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diff --git a/Documentation/ia64/mca.txt b/Documentation/ia64/mca.txt deleted file mode 100644 index f097c60cba1b..000000000000 --- a/Documentation/ia64/mca.txt +++ /dev/null @@ -1,194 +0,0 @@ -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 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 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. |