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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 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.