|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. 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".
| 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
|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
|How do we get a backtrace on the tasks that were running when MCA/INIT
|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
|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