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docs: Add spec of OVMF GUIDed table for SEV guests
Add docs/specs/sev-guest-firmware.rst which describes the GUIDed table in the end of OVMF's image which is parsed by QEMU, and currently used to describe some values for SEV and SEV-ES guests. Signed-off-by: Dov Murik <dovmurik@linux.ibm.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Message-Id: <20220103091413.2869-1-dovmurik@linux.ibm.com> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
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@@ -18,3 +18,4 @@ guest hardware that is specific to QEMU. | |
acpi_mem_hotplug | ||
acpi_pci_hotplug | ||
acpi_nvdimm | ||
sev-guest-firmware |
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==================================================== | ||
QEMU/Guest Firmware Interface for AMD SEV and SEV-ES | ||
==================================================== | ||
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Overview | ||
======== | ||
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The guest firmware image (OVMF) may contain some configuration entries | ||
which are used by QEMU before the guest launches. These are listed in a | ||
GUIDed table at a known location in the firmware image. QEMU parses | ||
this table when it loads the firmware image into memory, and then QEMU | ||
reads individual entries when their values are needed. | ||
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Though nothing in the table structure is SEV-specific, currently all the | ||
entries in the table are related to SEV and SEV-ES features. | ||
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Table parsing in QEMU | ||
--------------------- | ||
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The table is parsed from the footer: first the presence of the table | ||
footer GUID (96b582de-1fb2-45f7-baea-a366c55a082d) at 0xffffffd0 is | ||
verified. If that is found, two bytes at 0xffffffce are the entire | ||
table length. | ||
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Then the table is scanned backwards looking for the specific entry GUID. | ||
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QEMU files related to parsing and scanning the OVMF table: | ||
- ``hw/i386/pc_sysfw_ovmf.c`` | ||
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The edk2 firmware code that constructs this structure is in the | ||
`OVMF Reset Vector file`_. | ||
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Table memory layout | ||
------------------- | ||
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+------------+--------+-----------------------------------------+ | ||
| GPA | Length | Description | | ||
+============+========+=========================================+ | ||
| 0xffffff80 | 4 | Zero padding | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffff84 | 4 | SEV hashes table base address | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffff88 | 4 | SEV hashes table size (=0x400) | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffff8c | 2 | SEV hashes table entry length (=0x1a) | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffff8e | 16 | SEV hashes table GUID: | | ||
| | | 7255371f-3a3b-4b04-927b-1da6efa8d454 | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffff9e | 4 | SEV secret block base address | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffa2 | 4 | SEV secret block size (=0xc00) | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffa6 | 2 | SEV secret block entry length (=0x1a) | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffa8 | 16 | SEV secret block GUID: | | ||
| | | 4c2eb361-7d9b-4cc3-8081-127c90d3d294 | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffb8 | 4 | SEV-ES AP reset RIP | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffbc | 2 | SEV-ES reset block entry length (=0x16) | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffbe | 16 | SEV-ES reset block entry GUID: | | ||
| | | 00f771de-1a7e-4fcb-890e-68c77e2fb44e | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffce | 2 | Length of entire table including table | | ||
| | | footer GUID and length (=0x72) | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffd0 | 16 | OVMF GUIDed table footer GUID: | | ||
| | | 96b582de-1fb2-45f7-baea-a366c55a082d | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffe0 | 8 | Application processor entry point code | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xffffffe8 | 8 | "\0\0\0\0VTF\0" | | ||
+------------+--------+-----------------------------------------+ | ||
| 0xfffffff0 | 16 | Reset vector code | | ||
+------------+--------+-----------------------------------------+ | ||
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Table entries description | ||
========================= | ||
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SEV-ES reset block | ||
------------------ | ||
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Entry GUID: 00f771de-1a7e-4fcb-890e-68c77e2fb44e | ||
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For the initial boot of an AP under SEV-ES, the "reset" RIP must be | ||
programmed to the RAM area defined by this entry. The entry's format | ||
is: | ||
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* IP value [0:15] | ||
* CS segment base [31:16] | ||
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A hypervisor reads the CS segment base and IP value. The CS segment | ||
base value represents the high order 16-bits of the CS segment base, so | ||
the hypervisor must left shift the value of the CS segment base by 16 | ||
bits to form the full CS segment base for the CS segment register. It | ||
would then program the EIP register with the IP value as read. | ||
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SEV secret block | ||
---------------- | ||
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Entry GUID: 4c2eb361-7d9b-4cc3-8081-127c90d3d294 | ||
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This describes the guest RAM area where the hypervisor should inject the | ||
Guest Owner secret (using SEV_LAUNCH_SECRET). | ||
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SEV hashes table | ||
---------------- | ||
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Entry GUID: 7255371f-3a3b-4b04-927b-1da6efa8d454 | ||
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This describes the guest RAM area where the hypervisor should install a | ||
table describing the hashes of certain firmware configuration device | ||
files that would otherwise be passed in unchecked. The current use is | ||
for the kernel, initrd and command line values, but others may be added. | ||
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.. _OVMF Reset Vector file: | ||
https://github.com/tianocore/edk2/blob/master/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm |