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Decoding Intel(R) Processor Trace Using libipt {#libipt}

This chapter describes how to use libipt for various tasks around Intel Processor Trace (Intel PT). For code examples, refer to the sample tools that are contained in the source tree:

  • ptdump A packet dumper example.
  • ptxed A control-flow reconstruction example.
  • pttc A packet encoder example.

For detailed information about Intel PT, please refer to the respective chapter in Volume 3 of the Intel Software Developer's Manual at http://www.intel.com/sdm.

Introduction

The libipt decoder library provides multiple layers of abstraction ranging from packet encoding and decoding to full execution flow reconstruction. The layers are organized as follows:

  • packets This layer deals with raw Intel PT packets.

  • events This layer deals with packet combinations that encode higher-level events.

  • instruction flow This layer deals with the execution flow on the instruction level.

  • block This layer deals with the execution flow on the instruction level.

                        It is faster than the instruction flow decoder but
                        requires a small amount of post-processing.
    

Each layer provides its own encoder or decoder struct plus a set of functions for allocating and freeing encoder or decoder objects and for synchronizing decoders onto the Intel PT packet stream. Function names are prefixed with pt_<lyr>_ where <lyr> is an abbreviation of the layer name. The following abbreviations are used:

  • enc Packet encoding (packet layer).
  • pkt Packet decoding (packet layer).
  • qry Event (or query) layer.
  • insn Instruction flow layer.
  • blk Block layer.

Here is some generic example code for working with decoders:

    struct pt_<layer>_decoder *decoder;
    struct pt_config config;
    int errcode;

    memset(&config, 0, sizeof(config));
    config.size = sizeof(config);
    config.begin = <pt buffer begin>;
    config.end = <pt buffer end>;
    config.cpu = <cpu identifier>;
    config...

    decoder = pt_<lyr>_alloc_decoder(&config);
    if (!decoder)
        <handle error>(errcode);

    errcode = pt_<lyr>_sync_<where>(decoder);
    if (errcode < 0)
        <handle error>(errcode);

    <use decoder>(decoder);

    pt_<lyr>_free_decoder(decoder);

First, configure the decoder. As a minimum, the size of the config struct and the begin and end of the buffer containing the Intel PT data need to be set. Configuration options details will be discussed later in this chapter. In the case of packet encoding, this is the begin and end address of the pre-allocated buffer, into which Intel PT packets shall be written.

Next, allocate a decoder object for the layer you are interested in. A return value of NULL indicates an error. There is no further information available on the exact error condition. Most of the time, however, the error is the result of an incomplete or inconsistent configuration.

Before the decoder can be used, it needs to be synchronized onto the Intel PT packet stream specified in the configuration. The only exception to this is the packet encoder, which is implicitly synchronized onto the beginning of the Intel PT buffer.

Depending on the type of decoder, one or more synchronization options are available.

  • pt_<lyr>_sync_forward() Synchronize onto the next PSB in forward direction (or the first PSB if not yet synchronized).

  • pt_<lyr>_sync_backward() Synchronize onto the next PSB in backward direction (or the last PSB if not yet synchronized).

  • pt_<lyr>_sync_set() Set the synchronization position to a user-defined location in the Intel PT packet stream. There is no check whether the specified location makes sense or is valid.

After synchronizing, the decoder can be used. While decoding, the decoder stores the location of the last PSB it encountered during normal decode. Subsequent calls to pt__sync_forward() will start searching from that location. This is useful for re-synchronizing onto the Intel PT packet stream in case of errors. An example of a typical decode loop is given below:

    for (;;) {
        int errcode;

        errcode = <use decoder>(decoder);
        if (errcode >= 0)
            continue;

        if (errcode == -pte_eos)
            return;

        <report error>(errcode);

        do {
            errcode = pt_<lyr>_sync_forward(decoder);

            if (errcode == -pte_eos)
                return;
        } while (errcode < 0);
    }

You can get the current decoder position as offset into the Intel PT buffer via:

pt_<lyr>_get_offset()

You can get the position of the last synchronization point as offset into the Intel PT buffer via:

pt_<lyr>_get_sync_offset()

Each layer will be discussed in detail below. In the remainder of this section, general functionality will be considered.

Version

You can query the library version using:

  • pt_library_version()

This function returns a version structure that can be used for compatibility checks or simply for reporting the version of the decoder library.

Errors

The library uses a single error enum for all layers.

  • enum pt_error_code An enumeration of encode and decode errors.

Errors are typically represented as negative pt_error_code enumeration constants and returned as an int. The library provides two functions for dealing with errors:

  • pt_errcode() Translate an int return value into a pt_error_code enumeration constant.

  • pt_errstr() Returns a human-readable error string.

Not all errors may occur on every layer. Every API function specifies the errors it may return.

Configuration

Every encoder or decoder allocation function requires a configuration argument. Some of its fields have already been discussed in the example above. Refer to the intel-pt.h header for detailed and up-to-date documentation of each field.

As a minimum, the size field needs to be set to sizeof(struct pt_config) and begin and end need to be set to the Intel PT buffer to use.

The size is used for detecting library version mismatches and to provide backwards compatibility. Without the proper size, decoder allocation will fail.

Although not strictly required, it is recommended to also set the cpu field to the processor, on which Intel PT has been collected (for decoders), or for which Intel PT shall be generated (for encoders). This allows implementing processor-specific behavior such as erratum workarounds.

The Packet Layer

This layer deals with Intel PT packet encoding and decoding. It can further be split into three sub-layers: opcodes, encoding, and decoding.

Opcodes

The opcodes layer provides enumerations for all the bits necessary for Intel PT encoding and decoding. The enumeration constants can be used without linking to the decoder library. There is no encoder or decoder struct associated with this layer. See the intel-pt.h header file for details.

Packet Encoding

The packet encoding layer provides support for encoding Intel PT packet-by-packet. Start by configuring and allocating a pt_packet_encoder as shown below:

    struct pt_encoder *encoder;
    struct pt_config config;
    int errcode;

    memset(&config, 0, sizeof(config));
    config.size = sizeof(config);
    config.begin = <pt buffer begin>;
    config.end = <pt buffer end>;
    config.cpu = <cpu identifier>;

    encoder = pt_alloc_encoder(&config);
    if (!encoder)
        <handle error>(errcode);

For packet encoding, only the mandatory config fields need to be filled in.

The allocated encoder object will be implicitly synchronized onto the beginning of the Intel PT buffer. You may change the encoder's position at any time by calling pt_enc_sync_set() with the desired buffer offset.

Next, fill in a pt_packet object with details about the packet to be encoded. You do not need to fill in the size field. The needed size is computed by the encoder. There is no consistency check with the size specified in the packet object. The following example encodes a TIP packet:

    struct pt_packet_encoder *encoder = ...;
    struct pt_packet packet;
    int errcode;

    packet.type = ppt_tip;
    packet.payload.ip.ipc = pt_ipc_update_16;
    packet.payload.ip.ip = <ip>;

For IP packets, for example FUP or TIP.PGE, there is no need to mask out bits in the ip field that will not be encoded in the packet due to the specified IP compression in the ipc field. The encoder will ignore them.

There are no consistency checks whether the specified IP compression in the ipc field is allowed in the current context or whether decode will result in the full IP specified in the ip field.

Once the packet object has been filled, it can be handed over to the encoder as shown here:

    errcode = pt_enc_next(encoder, &packet);
    if (errcode < 0)
        <handle error>(errcode);

The encoder will encode the packet, write it into the Intel PT buffer, and advance its position to the next byte after the packet. On a successful encode, it will return the number of bytes that have been written. In case of errors, nothing will be written and the encoder returns a negative error code.

Packet Decoding

The packet decoding layer provides support for decoding Intel PT packet-by-packet. Start by configuring and allocating a pt_packet_decoder as shown here:

    struct pt_packet_decoder *decoder;
    struct pt_config config;
    int errcode;

    memset(&config, 0, sizeof(config));
    config.size = sizeof(config);
    config.begin = <pt buffer begin>;
    config.end = <pt buffer end>;
    config.cpu = <cpu identifier>;
    config.decode.callback = <decode function>;
    config.decode.context = <decode context>;

    decoder = pt_pkt_alloc_decoder(&config);
    if (!decoder)
        <handle error>(errcode);

For packet decoding, an optional decode callback function may be specified in addition to the mandatory config fields. If specified, the callback function will be called for packets the decoder does not know about. If there is no decode callback specified, the decoder will return -pte_bad_opc. In addition to the callback function pointer, an optional pointer to user-defined context information can be specified. This context will be passed to the decode callback function.

Before the decoder can be used, it needs to be synchronized onto the Intel PT packet stream. Packet decoders offer three synchronization functions. To iterate over synchronization points in the Intel PT packet stream in forward or backward direction, use one of the following two functions respectively:

pt_pkt_sync_forward()
pt_pkt_sync_backward()

To manually synchronize the decoder at a particular offset into the Intel PT packet stream, use the following function:

pt_pkt_sync_set()

There are no checks to ensure that the specified offset is at the beginning of a packet. The example below shows synchronization to the first synchronization point:

    struct pt_packet_decoder *decoder;
    int errcode;

    errcode = pt_pkt_sync_forward(decoder);
    if (errcode < 0)
        <handle error>(errcode);

The decoder will remember the last synchronization packet it decoded. Subsequent calls to pt_pkt_sync_forward and pt_pkt_sync_backward will use this as their starting point.

You can get the current decoder position as offset into the Intel PT buffer via:

pt_pkt_get_offset()

You can get the position of the last synchronization point as offset into the Intel PT buffer via:

pt_pkt_get_sync_offset()

Once the decoder is synchronized, you can iterate over packets by repeated calls to pt_pkt_next() as shown in the following example:

    struct pt_packet_decoder *decoder;
    int errcode;

    for (;;) {
        struct pt_packet packet;

        errcode = pt_pkt_next(decoder, &packet, sizeof(packet));
        if (errcode < 0)
            break;

        <process packet>(&packet);
    }

The Event Layer

The event layer deals with packet combinations that encode higher-level events. It is used for reconstructing execution flow for users who need finer-grain control not available via the instruction flow layer or for users who want to integrate execution flow reconstruction with other functionality more tightly than it would be possible otherwise.

This section describes how to use the query decoder for reconstructing execution flow. See the instruction flow decoder as an example. Start by configuring and allocating a pt_query_decoder as shown below:

    struct pt_query_decoder *decoder;
    struct pt_config config;
    int errcode;

    memset(&config, 0, sizeof(config));
    config.size = sizeof(config);
    config.begin = <pt buffer begin>;
    config.end = <pt buffer end>;
    config.cpu = <cpu identifier>;
    config.decode.callback = <decode function>;
    config.decode.context = <decode context>;

    decoder = pt_qry_alloc_decoder(&config);
    if (!decoder)
        <handle error>(errcode);

An optional packet decode callback function may be specified in addition to the mandatory config fields. If specified, the callback function will be called for packets the decoder does not know about. The query decoder will ignore the unknown packet except for its size in order to skip it. If there is no decode callback specified, the decoder will abort with -pte_bad_opc. In addition to the callback function pointer, an optional pointer to user-defined context information can be specified. This context will be passed to the decode callback function.

Before the decoder can be used, it needs to be synchronized onto the Intel PT packet stream. To iterate over synchronization points in the Intel PT packet stream in forward or backward direction, the query decoders offer the following two synchronization functions respectively:

pt_qry_sync_forward()
pt_qry_sync_backward()

To manually synchronize the decoder at a synchronization point (i.e. PSB packet) in the Intel PT packet stream, use the following function:

pt_qry_sync_set()

After successfully synchronizing, the query decoder will start reading the PSB+ header to initialize its internal state. If tracing is enabled at this synchronization point, the IP of the instruction, at which decoding should be started, is returned. If tracing is disabled at this synchronization point, it will be indicated in the returned status bits (see below). In this example, synchronization to the first synchronization point is shown:

    struct pt_query_decoder *decoder;
    uint64_t ip;
    int status;

    status = pt_qry_sync_forward(decoder, &ip);
    if (status < 0)
        <handle error>(status);

In addition to a query decoder, you will need an instruction decoder for decoding and classifying instructions.

In A Nutshell

After synchronizing, you begin decoding instructions starting at the returned IP. As long as you can determine the next instruction in execution order, you continue on your own. Only when the next instruction cannot be determined by examining the current instruction, you would ask the query decoder for guidance:

  • If the current instruction is a conditional branch, the pt_qry_cond_branch() function will tell whether it was taken.

  • If the current instruction is an indirect branch, the pt_qry_indirect_branch() function will provide the IP of its destination.

    struct pt_query_decoder *decoder;
    uint64_t ip;

    for (;;) {
        struct <instruction> insn;

        insn = <decode instruction>(ip);

        ip += <instruction size>(insn);

        if (<is cond branch>(insn)) {
            int status, taken;

            status = pt_qry_cond_branch(decoder, &taken);
            if (status < 0)
                <handle error>(status);

            if (taken)
                ip += <branch displacement>(insn);
        } else if (<is indirect branch>(insn)) {
            int status;

            status = pt_qry_indirect_branch(decoder, &ip);
            if (status < 0)
                <handle error>(status);
        }
    }

Certain aspects such as, for example, asynchronous events or synchronizing at a location where tracing is disabled, have been ignored so far. Let us consider them now.

Queries

The query decoder provides four query functions:

  • pt_qry_cond_branch() Query whether the next conditional branch was taken.

  • pt_qry_indirect_branch() Query for the destination IP of the next indirect branch.

  • pt_qry_event() Query for the next event.

  • pt_qry_time() Query for the current time.

Each function returns either a positive vector of status bits or a negative error code. For details on status bits and error conditions, please refer to the pt_status_flag and pt_error_code enumerations in the intel-pt.h header.

The pts_ip_suppressed status bit is used to indicate that no IP is available at functions that are supposed to return an IP. Examples are the indirect branch query function and both synchronization functions.

The pts_event_pending status bit is used to indicate that there is an event pending. You should query for this event before continuing execution flow reconstruction.

The pts_eos status bit is used to indicate the end of the trace. Any subsequent query will return -pte_eos.

Events

Events are signaled ahead of time. When you query for pending events as soon as they are indicated, you will be aware of asynchronous events before you reach the instruction associated with the event.

For example, if tracing is disabled at the synchronization point, the IP will be suppressed. In this case, it is very likely that a tracing enabled event is signaled. You will also get events for initializing the decoder state after synchronizing onto the Intel PT packet stream. For example, paging or execution mode events.

See the enum pt_event_type and struct pt_event in the intel-pt.h header for details on possible events. This document does not give an example of event processing. Refer to the implementation of the instruction flow decoder in pt_insn.c for details.

Timing

To be able to signal events, the decoder reads ahead until it arrives at a query relevant packet. Errors encountered during that time will be postponed until the respective query call. This reading ahead affects timing. The decoder will always be a few packets ahead. When querying for the current time, the query will return the time at the decoder's current packet. This corresponds to the time at our next query.

Return Compression

If Intel PT has been configured to compress returns, a successfully compressed return is represented as a conditional branch instead of an indirect branch. For a RET instruction, you first query for a conditional branch. If the query succeeds, it should indicate that the branch was taken. In that case, the return has been compressed. A not taken branch indicates an error. If the query fails, the return has not been compressed and you query for an indirect branch.

There is no guarantee that returns will be compressed. Even though return compression has been enabled, returns may still be represented as indirect branches.

To reconstruct the execution flow for compressed returns, you would maintain a stack of return addresses. For each call instruction, push the IP of the instruction following the call onto the stack. For compressed returns, pop the topmost IP from the stack. See pt_retstack.h and pt_retstack.c for a sample implementation.

The Instruction Flow Layer

The instruction flow layer provides a simple API for iterating over instructions in execution order. Start by configuring and allocating a pt_insn_decoder as shown below:

    struct pt_insn_decoder *decoder;
    struct pt_config config;
    int errcode;

    memset(&config, 0, sizeof(config));
    config.size = sizeof(config);
    config.begin = <pt buffer begin>;
    config.end = <pt buffer end>;
    config.cpu = <cpu identifier>;
    config.decode.callback = <decode function>;
    config.decode.context = <decode context>;

    decoder = pt_insn_alloc_decoder(&config);
    if (!decoder)
        <handle error>(errcode);

An optional packet decode callback function may be specified in addition to the mandatory config fields. If specified, the callback function will be called for packets the decoder does not know about. The decoder will ignore the unknown packet except for its size in order to skip it. If there is no decode callback specified, the decoder will abort with -pte_bad_opc. In addition to the callback function pointer, an optional pointer to user-defined context information can be specified. This context will be passed to the decode callback function.

The image argument is optional. If no image is given, the decoder will use an empty default image that can be populated later on and that is implicitly destroyed when the decoder is freed. See below for more information on this.

The Traced Image

In addition to the Intel PT configuration, the instruction flow decoder needs to know the memory image for which Intel PT has been recorded. This memory image is represented by a pt_image object. If decoding failed due to an IP lying outside of the traced memory image, pt_insn_next() will return -pte_nomap.

Use pt_image_alloc() to allocate and pt_image_free() to free an image. Images may not be shared. Every decoder must use a different image. Use this to prepare the image in advance or if you want to switch between images.

Every decoder provides an empty default image that is used if no image is specified during allocation. The default image is implicitly destroyed when the decoder is freed. It can be obtained by calling pt_insn_get_image(). Use this if you only use one decoder and one image.

An image is a collection of contiguous, non-overlapping memory regions called sections. Starting with an empty image, it may be populated with repeated calls to pt_image_add_file() or pt_image_add_cached(), one for each section, or with a call to pt_image_copy() to add all sections from another image. If a newly added section overlaps with an existing section, the existing section will be truncated or split to make room for the new section.

In some cases, the memory image may change during the execution. You can use the pt_image_remove_by_filename() function to remove previously added sections by their file name and pt_image_remove_by_asid() to remove all sections for an address-space.

In addition to adding sections, you can register a callback function for reading memory using pt_image_set_callback(). The context parameter you pass together with the callback function pointer will be passed to your callback function every time it is called. There can only be one callback at any time. Adding a new callback will remove any previously added callback. To remove the callback function, pass NULL to pt_image_set_callback().

Callback and files may be combined. The callback function is used whenever the memory cannot be found in any of the image's sections.

If more than one process is traced, the memory image may change when the process context is switched. To simplify handling this case, an address-space identifier may be passed to each of the above functions to define separate images for different processes at the same time. The decoder will select the correct image based on context switch information in the Intel PT trace. If you want to manage this on your own, you can use pt_insn_set_image() to replace the image a decoder uses.

The Traced Image Section Cache

When using multiple decoders that work on related memory images it is desirable to share image sections between decoders. The underlying file sections will be mapped only once per image section cache.

Use pt_iscache_alloc() to allocate and pt_iscache_free() to free an image section cache. Freeing the cache does not destroy sections added to the cache. They remain valid until they are no longer used.

Use pt_iscache_add_file() to add a file section to an image section cache. The function returns an image section identifier (ISID) that uniquely identifies the section in this cache. Use pt_image_add_cached() to add a file section from an image section cache to an image.

Multiple image section caches may be used at the same time but it is recommended not to mix sections from different image section caches in one image.

A traced image section cache can also be used for reading an instruction's memory via its IP and ISID as provided in struct pt_insn.

The image section cache provides a cache of recently mapped sections and keeps them mapped when they are unmapped by the images that used them. This avoid repeated unmapping and re-mapping of image sections in some parallel debug scenarios or when reading memory from the image section cache.

Use pt_iscache_set_limit() to set the limit of this cache in bytes. This accounts for the extra memory that will be used for keeping image sections mapped including any block caches associated with image sections. To disable caching, set the limit to zero.

Synchronizing

Before the decoder can be used, it needs to be synchronized onto the Intel PT packet stream. To iterate over synchronization points in the Intel PT packet stream in forward or backward directions, the instruction flow decoders offer the following two synchronization functions respectively:

pt_insn_sync_forward()
pt_insn_sync_backward()

To manually synchronize the decoder at a synchronization point (i.e. PSB packet) in the Intel PT packet stream, use the following function:

pt_insn_sync_set()

The example below shows synchronization to the first synchronization point:

    struct pt_insn_decoder *decoder;
    int errcode;

    errcode = pt_insn_sync_forward(decoder);
    if (errcode < 0)
        <handle error>(errcode);

The decoder will remember the last synchronization packet it decoded. Subsequent calls to pt_insn_sync_forward and pt_insn_sync_backward will use this as their starting point.

You can get the current decoder position as offset into the Intel PT buffer via:

pt_insn_get_offset()

You can get the position of the last synchronization point as offset into the Intel PT buffer via:

pt_insn_get_sync_offset()

Iterating

Once the decoder is synchronized, you can iterate over instructions in execution flow order by repeated calls to pt_insn_next() as shown in the following example:

    struct pt_insn_decoder *decoder;
    int status;

    for (;;) {
        struct pt_insn insn;

        status = pt_insn_next(decoder, &insn, sizeof(insn));

        if (insn.iclass != ptic_error)
            <process instruction>(&insn);

        if (status < 0)
            break;

        ...
    }

Note that the example ignores non-error status returns.

For each instruction, you get its IP, its size in bytes, the raw memory, an identifier for the image section that contained it, the current execution mode, and the speculation state, that is whether the instruction has been executed speculatively. In addition, you get a coarse classification that can be used for further processing without the need for a full instruction decode.

If a traced image section cache is used the image section identifier can be used to trace an instruction back to the binary file that contained it. This allows mapping the instruction back to source code using the debug information contained in or reachable via the binary file.

Beware that pt_insn_next() may indicate errors that occur after the returned instruction. The returned instruction is valid if its iclass field is set.

Events

The instruction flow decoder uses an event system similar to the query decoder's. Pending events are indicated by the pts_event_pending flag in the status flag bit-vector returned from pt_insn_sync_<where>(), pt_insn_next() and pt_insn_event().

When the pts_event_pending flag is set on return from pt_insn_next(), use repeated calls to pt_insn_event() to drain all queued events. Then switch back to calling pt_insn_next() to resume with instruction flow decode as shown in the following example:

    struct pt_insn_decoder *decoder;
    int status;

    for (;;) {
        struct pt_insn insn;

        status = pt_insn_next(decoder, &insn, sizeof(insn));
        if (status < 0)
            break;

        <process instruction>(&insn);

        while (status & pts_event_pending) {
            struct pt_event event;

            status = pt_insn_event(decoder, &event, sizeof(event));
            if (status < 0)
                <handle error>(status);

            <process event>(&event);
        }
    }

The Instruction Flow Decode Loop

If we put all of the above examples together, we end up with a decode loop as shown below:

    int handle_events(struct pt_insn_decoder *decoder, int status)
    {
        while (status & pts_event_pending) {
            struct pt_event event;

            status = pt_insn_event(decoder, &event, sizeof(event));
            if (status < 0)
                break;

            <process event>(&event);
        }

        return status;
    }

    int decode(struct pt_insn_decoder *decoder)
    {
        int status;

        for (;;) {
            status = pt_insn_sync_forward(decoder);
            if (status < 0)
                break;

            for (;;) {
                struct pt_insn insn;

                status = handle_events(decoder, status);
                if (status < 0)
                    break;

                status = pt_insn_next(decoder, &insn, sizeof(insn));

                if (insn.iclass != ptic_error)
                    <process instruction>(&insn);

                if (status < 0)
                    break;
            }

            <handle error>(status);
        }

        <handle error>(status);

        return status;
    }

The Block Layer

The block layer provides a simple API for iterating over blocks of sequential instructions in execution order. The instructions in a block are sequential in the sense that no trace is required for reconstructing the instructions. The IP of the first instruction is given in struct pt_block and the IP of other instructions in the block can be determined by decoding and examining the previous instruction.

Start by configuring and allocating a pt_block_decoder as shown below:

    struct pt_block_decoder *decoder;
    struct pt_config config;

    memset(&config, 0, sizeof(config));
    config.size = sizeof(config);
    config.begin = <pt buffer begin>;
    config.end = <pt buffer end>;
    config.cpu = <cpu identifier>;
    config.decode.callback = <decode function>;
    config.decode.context = <decode context>;

    decoder = pt_blk_alloc_decoder(&config);

An optional packet decode callback function may be specified in addition to the mandatory config fields. If specified, the callback function will be called for packets the decoder does not know about. The decoder will ignore the unknown packet except for its size in order to skip it. If there is no decode callback specified, the decoder will abort with -pte_bad_opc. In addition to the callback function pointer, an optional pointer to user-defined context information can be specified. This context will be passed to the decode callback function.

Synchronizing

Before the decoder can be used, it needs to be synchronized onto the Intel PT packet stream. To iterate over synchronization points in the Intel PT packet stream in forward or backward directions, the block decoder offers the following two synchronization functions respectively:

pt_blk_sync_forward()
pt_blk_sync_backward()

To manually synchronize the decoder at a synchronization point (i.e. PSB packet) in the Intel PT packet stream, use the following function:

pt_blk_sync_set()

The example below shows synchronization to the first synchronization point:

    struct pt_block_decoder *decoder;
    int errcode;

    errcode = pt_blk_sync_forward(decoder);
    if (errcode < 0)
        <handle error>(errcode);

The decoder will remember the last synchronization packet it decoded. Subsequent calls to pt_blk_sync_forward and pt_blk_sync_backward will use this as their starting point.

You can get the current decoder position as offset into the Intel PT buffer via:

pt_blk_get_offset()

You can get the position of the last synchronization point as offset into the Intel PT buffer via:

pt_blk_get_sync_offset()

Iterating

Once the decoder is synchronized, it can be used to iterate over blocks of instructions in execution flow order by repeated calls to pt_blk_next() as shown in the following example:

    struct pt_block_decoder *decoder;
    int status;

    for (;;) {
        struct pt_block block;

        status = pt_blk_next(decoder, &block, sizeof(block));

        if (block.ninsn > 0)
            <process block>(&block);

        if (status < 0)
            break;

        ...
    }

Note that the example ignores non-error status returns.

A block contains enough information to reconstruct the instructions. See struct pt_block in intel-pt.h for details. Note that errors returned by pt_blk_next() apply after the last instruction in the provided block.

It is recommended to use a traced image section cache so the image section identifier contained in a block can be used for reading the memory containing the instructions in the block. This also allows mapping the instructions back to source code using the debug information contained in or reachable via the binary file.

In some cases, the last instruction in a block may cross image section boundaries. This can happen when a code segment is split into more than one image section. The block is marked truncated in this case and provides the raw bytes of the last instruction.

The following example shows how instructions can be reconstructed from a block:

    struct pt_image_section_cache *iscache;
    struct pt_block *block;
    uint16_t ninsn;
    uint64_t ip;

    ip = block->ip;
    for (ninsn = 0; ninsn < block->ninsn; ++ninsn) {
        uint8_t raw[pt_max_insn_size];
        <struct insn> insn;
        int size;

        if (block->truncated && ((ninsn +1) == block->ninsn)) {
            memcpy(raw, block->raw, block->size);
            size = block->size;
        } else {
            size = pt_iscache_read(iscache, raw, sizeof(raw), block->isid, ip);
            if (size < 0)
                break;
        }

        errcode = <decode instruction>(&insn, raw, size, block->mode);
        if (errcode < 0)
            break;

        <process instruction>(&insn);

        ip = <determine next ip>(&insn);
    }

Events

The block decoder uses an event system similar to the query decoder's. Pending events are indicated by the pts_event_pending flag in the status flag bit-vector returned from pt_blk_sync_<where>(), pt_blk_next() and pt_blk_event().

When the pts_event_pending flag is set on return from pt_blk_sync_<where>() or pt_blk_next(), use repeated calls to pt_blk_event() to drain all queued events. Then switch back to calling pt_blk_next() to resume with block decode as shown in the following example:

    struct pt_block_decoder *decoder;
    int status;

    for (;;) {
        struct pt_block block;

        status = pt_blk_next(decoder, &block, sizeof(block));
        if (status < 0)
            break;

        <process block>(&block);

        while (status & pts_event_pending) {
            struct pt_event event;

            status = pt_blk_event(decoder, &event, sizeof(event));
            if (status < 0)
                <handle error>(status);

            <process event>(&event);
        }
    }

The Block Decode Loop

If we put all of the above examples together, we end up with a decode loop as shown below:

    int process_block(struct pt_block *block,
                      struct pt_image_section_cache *iscache)
    {
        uint16_t ninsn;
        uint64_t ip;

        ip = block->ip;
        for (ninsn = 0; ninsn < block->ninsn; ++ninsn) {
            struct pt_insn insn;

            memset(&insn, 0, sizeof(insn));
            insn->speculative = block->speculative;
            insn->isid = block->isid;
            insn->mode = block->mode;
            insn->ip = ip;

            if (block->truncated && ((ninsn +1) == block->ninsn)) {
                insn.truncated = 1;
                insn.size = block->size;

                memcpy(insn.raw, block->raw, insn.size);
            } else {
                int size;

                size = pt_iscache_read(iscache, insn.raw, sizeof(insn.raw),
                                       insn.isid, insn.ip);
                if (size < 0)
                    return size;

                insn.size = (uint8_t) size;
            }

            <decode instruction>(&insn);
            <process instruction>(&insn);

            ip = <determine next ip>(&insn);
        }

        return 0;
    }

    int handle_events(struct pt_blk_decoder *decoder, int status)
    {
        while (status & pts_event_pending) {
            struct pt_event event;

            status = pt_blk_event(decoder, &event, sizeof(event));
            if (status < 0)
                break;

            <process event>(&event);
        }

        return status;
    }

    int decode(struct pt_blk_decoder *decoder,
               struct pt_image_section_cache *iscache)
    {
        int status;

        for (;;) {
            status = pt_blk_sync_forward(decoder);
            if (status < 0)
                break;

            for (;;) {
                struct pt_block block;
                int errcode;

                status = handle_events(decoder, status);
                if (status < 0)
                    break;

                status = pt_blk_next(decoder, &block, sizeof(block));

                errcode = process_block(&block, iscache);
                if (errcode < 0)
                    status = errcode;

                if (status < 0)
                    break;
            }

            <handle error>(status);
        }

        <handle error>(status);

        return status;
    }

Parallel Decode

Intel PT splits naturally into self-contained PSB segments that can be decoded independently. Use the packet or query decoder to search for PSB's using repeated calls to pt_pkt_sync_forward() and pt_pkt_get_sync_offset() (or pt_qry_sync_forward() and pt_qry_get_sync_offset()). The following example shows this using the query decoder, which will already give the IP needed in the next step.

    struct pt_query_decoder *decoder;
    uint64_t offset, ip;
    int status, errcode;

    for (;;) {
        status = pt_qry_sync_forward(decoder, &ip);
        if (status < 0)
            break;

        errcode = pt_qry_get_sync_offset(decoder, &offset);
        if (errcode < 0)
            <handle error>(errcode);

        <split trace>(offset, ip, status);
    }

The individual trace segments can then be decoded using the query, instruction flow, or block decoder as shown above in the previous examples.

When stitching decoded trace segments together, a sequence of linear (in the sense that it can be decoded without Intel PT) code has to be filled in. Use the pts_eos status indication to stop decoding early enough. Then proceed until the IP at the start of the succeeding trace segment is reached. When using the instruction flow decoder, pt_insn_next() may be used for that as shown in the following example:

    struct pt_insn_decoder *decoder;
    struct pt_insn insn;
    int status;

    for (;;) {
        status = pt_insn_next(decoder, &insn, sizeof(insn));
        if (status < 0)
            <handle error>(status);

        if (status & pts_eos)
            break;

        <process instruction>(&insn);
    }

    while (insn.ip != <next segment's start IP>) {
        <process instruction>(&insn);

        status = pt_insn_next(decoder, &insn, sizeof(insn));
        if (status < 0)
            <handle error>(status);
    }

Threading

The decoder library API is not thread-safe. Different threads may allocate and use different decoder objects at the same time. Different decoders must not use the same image object. Use pt_image_copy() to give each decoder its own copy of a shared master image.

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