Adding upstream version 6.7.7.upstream/6.7.7

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 0 insertions, 198 deletions

diff --git a/Documentation/arch/ia64/mca.rst b/Documentation/arch/ia64/mca.rst
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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.