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| Original file line number | Diff line number | Diff line change |
|---|---|---|
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| An ad-hoc collection of notes on IA64 MCA and INIT processing. Feel | ||
| free to update it with notes about any area that is not clear. | ||
|
|
||
| --- | ||
|
|
||
| MCA/INIT are completely asynchronous. They can occur at any time, when | ||
| the OS is in any state. Including when one of the cpus is already | ||
| holding a spinlock. Trying to get any lock from MCA/INIT state is | ||
| asking for deadlock. Also the state of structures that are protected | ||
| by locks is indeterminate, including linked lists. | ||
|
|
||
| --- | ||
|
|
||
| The complicated ia64 MCA process. All of this is mandated by Intel's | ||
| specification for ia64 SAL, error recovery and and unwind, it is not as | ||
| if we have a choice here. | ||
|
|
||
| * MCA occurs on one cpu, usually due to a double bit memory error. | ||
| This is the monarch cpu. | ||
|
|
||
| * SAL sends an MCA rendezvous interrupt (which is a normal interrupt) | ||
| to all the other cpus, the slaves. | ||
|
|
||
| * Slave cpus that receive the MCA interrupt call down into SAL, they | ||
| end up spinning disabled while the MCA is being serviced. | ||
|
|
||
| * If any slave cpu was already spinning disabled when the MCA occurred | ||
| then it cannot service the MCA interrupt. SAL waits ~20 seconds then | ||
| sends an unmaskable INIT event to the slave cpus that have not | ||
| already rendezvoused. | ||
|
|
||
| * Because MCA/INIT can be delivered at any time, including when the cpu | ||
| is down in PAL in physical mode, the registers at the time of the | ||
| event are _completely_ undefined. In particular the MCA/INIT | ||
| handlers cannot rely on the thread pointer, PAL physical mode can | ||
| (and does) modify TP. It is allowed to do that as long as it resets | ||
| TP on return. However MCA/INIT events expose us to these PAL | ||
| internal TP changes. Hence curr_task(). | ||
|
|
||
| * If an MCA/INIT event occurs while the kernel was running (not user | ||
| space) and the kernel has called PAL then the MCA/INIT handler cannot | ||
| assume that the kernel stack is in a fit state to be used. Mainly | ||
| because PAL may or may not maintain the stack pointer internally. | ||
| Because the MCA/INIT handlers cannot trust the kernel stack, they | ||
| have to use their own, per-cpu stacks. The MCA/INIT stacks are | ||
| preformatted with just enough task state to let the relevant handlers | ||
| do their job. | ||
|
|
||
| * Unlike most other architectures, the ia64 struct task is embedded in | ||
| the kernel stack[1]. So switching to a new kernel stack means that | ||
| we switch to a new task as well. Because various bits of the kernel | ||
| assume that current points into the struct task, switching to a new | ||
| stack also means a new value for current. | ||
|
|
||
| * Once all slaves have rendezvoused and are spinning disabled, the | ||
| monarch is entered. The monarch now tries to diagnose the problem | ||
| and decide if it can recover or not. | ||
|
|
||
| * Part of the monarch's job is to look at the state of all the other | ||
| tasks. The only way to do that on ia64 is to call the unwinder, | ||
| as mandated by Intel. | ||
|
|
||
| * The starting point for the unwind depends on whether a task is | ||
| running or not. That is, whether it is on a cpu or is blocked. The | ||
| monarch has to determine whether or not a task is on a cpu before it | ||
| knows how to start unwinding it. The tasks that received an MCA or | ||
| INIT event are no longer running, they have been converted to blocked | ||
| tasks. But (and its a big but), the cpus that received the MCA | ||
| rendezvous interrupt are still running on their normal kernel stacks! | ||
|
|
||
| * To distinguish between these two cases, the monarch must know which | ||
| tasks are on a cpu and which are not. Hence each slave cpu that | ||
| switches to an MCA/INIT stack, registers its new stack using | ||
| set_curr_task(), so the monarch can tell that the _original_ task is | ||
| no longer running on that cpu. That gives us a decent chance of | ||
| getting a valid backtrace of the _original_ task. | ||
|
|
||
| * MCA/INIT can be nested, to a depth of 2 on any cpu. In the case of a | ||
| nested error, we want diagnostics on the MCA/INIT handler that | ||
| failed, not on the task that was originally running. Again this | ||
| requires set_curr_task() so the MCA/INIT handlers can register their | ||
| own stack as running on that cpu. Then a recursive error gets a | ||
| trace of the failing handler's "task". | ||
|
|
||
| [1] My (Keith Owens) original design called for ia64 to separate its | ||
| struct task and the kernel stacks. Then the MCA/INIT data would be | ||
| chained stacks like i386 interrupt stacks. But that required | ||
| radical surgery on the rest of ia64, plus extra hard wired TLB | ||
| entries with its associated performance degradation. David | ||
| Mosberger vetoed that approach. Which meant that separate kernel | ||
| stacks meant separate "tasks" for the MCA/INIT handlers. | ||
|
|
||
| --- | ||
|
|
||
| INIT is less complicated than MCA. Pressing the nmi button or using | ||
| the equivalent command on the management console sends INIT to all | ||
| cpus. SAL picks one one of the cpus as the monarch and the rest are | ||
| slaves. All the OS INIT handlers are entered at approximately the same | ||
| time. The OS monarch prints the state of all tasks and returns, after | ||
| which the slaves return and the system resumes. | ||
|
|
||
| At least that is what is supposed to happen. Alas there are broken | ||
| versions of SAL out there. Some drive all the cpus as monarchs. Some | ||
| drive them all as slaves. Some drive one cpu as monarch, wait for that | ||
| cpu to return from the OS then drive the rest as slaves. Some versions | ||
| of SAL cannot even cope with returning from the OS, they spin inside | ||
| SAL on resume. The OS INIT code has workarounds for some of these | ||
| broken SAL symptoms, but some simply cannot be fixed from the OS side. | ||
|
|
||
| --- | ||
|
|
||
| The scheduler hooks used by ia64 (curr_task, set_curr_task) are layer | ||
| violations. Unfortunately MCA/INIT start off as massive layer | ||
| violations (can occur at _any_ time) and they build from there. | ||
|
|
||
| At least ia64 makes an attempt at recovering from hardware errors, but | ||
| it is a difficult problem because of the asynchronous nature of these | ||
| errors. When processing an unmaskable interrupt we sometimes need | ||
| special code to cope with our inability to take any locks. | ||
|
|
||
| --- | ||
|
|
||
| How is ia64 MCA/INIT different from x86 NMI? | ||
|
|
||
| * x86 NMI typically gets delivered to one cpu. MCA/INIT gets sent to | ||
| all cpus. | ||
|
|
||
| * x86 NMI cannot be nested. MCA/INIT can be nested, to a depth of 2 | ||
| per cpu. | ||
|
|
||
| * x86 has a separate struct task which points to one of multiple kernel | ||
| stacks. ia64 has the struct task embedded in the single kernel | ||
| stack, so switching stack means switching task. | ||
|
|
||
| * x86 does not call the BIOS so the NMI handler does not have to worry | ||
| about any registers having changed. MCA/INIT can occur while the cpu | ||
| is in PAL in physical mode, with undefined registers and an undefined | ||
| kernel stack. | ||
|
|
||
| * i386 backtrace is not very sensitive to whether a process is running | ||
| or not. ia64 unwind is very, very sensitive to whether a process is | ||
| running or not. | ||
|
|
||
| --- | ||
|
|
||
| What happens when MCA/INIT is delivered what a cpu is running user | ||
| space code? | ||
|
|
||
| The user mode registers are stored in the RSE area of the MCA/INIT on | ||
| entry to the OS and are restored from there on return to SAL, so user | ||
| mode registers are preserved across a recoverable MCA/INIT. Since the | ||
| OS has no idea what unwind data is available for the user space stack, | ||
| MCA/INIT never tries to backtrace user space. Which means that the OS | ||
| does not bother making the user space process look like a blocked task, | ||
| i.e. the OS does not copy pt_regs and switch_stack to the user space | ||
| stack. Also the OS has no idea how big the user space RSE and memory | ||
| stacks are, which makes it too risky to copy the saved state to a user | ||
| mode stack. | ||
|
|
||
| --- | ||
|
|
||
| How do we get a backtrace on the tasks that were running when MCA/INIT | ||
| was delivered? | ||
|
|
||
| mca.c:::ia64_mca_modify_original_stack(). That identifies and | ||
| verifies the original kernel stack, copies the dirty registers from | ||
| the MCA/INIT stack's RSE to the original stack's RSE, copies the | ||
| skeleton struct pt_regs and switch_stack to the original stack, fills | ||
| in the skeleton structures from the PAL minstate area and updates the | ||
| original stack's thread.ksp. That makes the original stack look | ||
| exactly like any other blocked task, i.e. it now appears to be | ||
| sleeping. To get a backtrace, just start with thread.ksp for the | ||
| original task and unwind like any other sleeping task. | ||
|
|
||
| --- | ||
|
|
||
| How do we identify the tasks that were running when MCA/INIT was | ||
| delivered? | ||
|
|
||
| If the previous task has been verified and converted to a blocked | ||
| state, then sos->prev_task on the MCA/INIT stack is updated to point to | ||
| the previous task. You can look at that field in dumps or debuggers. | ||
| To help distinguish between the handler and the original tasks, | ||
| handlers have _TIF_MCA_INIT set in thread_info.flags. | ||
|
|
||
| The sos data is always in the MCA/INIT handler stack, at offset | ||
| MCA_SOS_OFFSET. You can get that value from mca_asm.h or calculate it | ||
| as KERNEL_STACK_SIZE - sizeof(struct pt_regs) - sizeof(struct | ||
| ia64_sal_os_state), with 16 byte alignment for all structures. | ||
|
|
||
| Also the comm field of the MCA/INIT task is modified to include the pid | ||
| of the original task, for humans to use. For example, a comm field of | ||
| 'MCA 12159' means that pid 12159 was running when the MCA was | ||
| delivered. |
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