docs: move x86 documentation into Documentation/arch/

Move the x86 documentation under Documentation/arch/ as a way of cleaning
up the top-level directory and making the structure of our docs more
closely match the structure of the source directories it describes.

All in-kernel references to the old paths have been updated.

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-arch@vger.kernel.org
Cc: x86@kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/20230315211523.108836-1-corbet@lwn.net/
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 0 insertions, 152 deletions

diff --git a/Documentation/x86/kernel-stacks.rst b/Documentation/x86/kernel-stacks.rst
deleted file mode 100644
index 6b0bcf027ff1..000000000000
--- a/Documentation/x86/kernel-stacks.rst
+++ /dev/null
@@ -1,152 +0,0 @@
-.. SPDX-License-Identifier: GPL-2.0
-
-=============
-Kernel Stacks
-=============
-
-Kernel stacks on x86-64 bit
-===========================
-
-Most of the text from Keith Owens, hacked by AK
-
-x86_64 page size (PAGE_SIZE) is 4K.
-
-Like all other architectures, x86_64 has a kernel stack for every
-active thread.  These thread stacks are THREAD_SIZE (2*PAGE_SIZE) big.
-These stacks contain useful data as long as a thread is alive or a
-zombie. While the thread is in user space the kernel stack is empty
-except for the thread_info structure at the bottom.
-
-In addition to the per thread stacks, there are specialized stacks
-associated with each CPU.  These stacks are only used while the kernel
-is in control on that CPU; when a CPU returns to user space the
-specialized stacks contain no useful data.  The main CPU stacks are:
-
-* Interrupt stack.  IRQ_STACK_SIZE
-
-  Used for external hardware interrupts.  If this is the first external
-  hardware interrupt (i.e. not a nested hardware interrupt) then the
-  kernel switches from the current task to the interrupt stack.  Like
-  the split thread and interrupt stacks on i386, this gives more room
-  for kernel interrupt processing without having to increase the size
-  of every per thread stack.
-
-  The interrupt stack is also used when processing a softirq.
-
-Switching to the kernel interrupt stack is done by software based on a
-per CPU interrupt nest counter. This is needed because x86-64 "IST"
-hardware stacks cannot nest without races.
-
-x86_64 also has a feature which is not available on i386, the ability
-to automatically switch to a new stack for designated events such as
-double fault or NMI, which makes it easier to handle these unusual
-events on x86_64.  This feature is called the Interrupt Stack Table
-(IST).  There can be up to 7 IST entries per CPU. The IST code is an
-index into the Task State Segment (TSS). The IST entries in the TSS
-point to dedicated stacks; each stack can be a different size.
-
-An IST is selected by a non-zero value in the IST field of an
-interrupt-gate descriptor.  When an interrupt occurs and the hardware
-loads such a descriptor, the hardware automatically sets the new stack
-pointer based on the IST value, then invokes the interrupt handler.  If
-the interrupt came from user mode, then the interrupt handler prologue
-will switch back to the per-thread stack.  If software wants to allow
-nested IST interrupts then the handler must adjust the IST values on
-entry to and exit from the interrupt handler.  (This is occasionally
-done, e.g. for debug exceptions.)
-
-Events with different IST codes (i.e. with different stacks) can be
-nested.  For example, a debug interrupt can safely be interrupted by an
-NMI.  arch/x86_64/kernel/entry.S::paranoidentry adjusts the stack
-pointers on entry to and exit from all IST events, in theory allowing
-IST events with the same code to be nested.  However in most cases, the
-stack size allocated to an IST assumes no nesting for the same code.
-If that assumption is ever broken then the stacks will become corrupt.
-
-The currently assigned IST stacks are:
-
-* ESTACK_DF.  EXCEPTION_STKSZ (PAGE_SIZE).
-
-  Used for interrupt 8 - Double Fault Exception (#DF).
-
-  Invoked when handling one exception causes another exception. Happens
-  when the kernel is very confused (e.g. kernel stack pointer corrupt).
-  Using a separate stack allows the kernel to recover from it well enough
-  in many cases to still output an oops.
-
-* ESTACK_NMI.  EXCEPTION_STKSZ (PAGE_SIZE).
-
-  Used for non-maskable interrupts (NMI).
-
-  NMI can be delivered at any time, including when the kernel is in the
-  middle of switching stacks.  Using IST for NMI events avoids making
-  assumptions about the previous state of the kernel stack.
-
-* ESTACK_DB.  EXCEPTION_STKSZ (PAGE_SIZE).
-
-  Used for hardware debug interrupts (interrupt 1) and for software
-  debug interrupts (INT3).
-
-  When debugging a kernel, debug interrupts (both hardware and
-  software) can occur at any time.  Using IST for these interrupts
-  avoids making assumptions about the previous state of the kernel
-  stack.
-
-  To handle nested #DB correctly there exist two instances of DB stacks. On
-  #DB entry the IST stackpointer for #DB is switched to the second instance
-  so a nested #DB starts from a clean stack. The nested #DB switches
-  the IST stackpointer to a guard hole to catch triple nesting.
-
-* ESTACK_MCE.  EXCEPTION_STKSZ (PAGE_SIZE).
-
-  Used for interrupt 18 - Machine Check Exception (#MC).
-
-  MCE can be delivered at any time, including when the kernel is in the
-  middle of switching stacks.  Using IST for MCE events avoids making
-  assumptions about the previous state of the kernel stack.
-
-For more details see the Intel IA32 or AMD AMD64 architecture manuals.
-
-
-Printing backtraces on x86
-==========================
-
-The question about the '?' preceding function names in an x86 stacktrace
-keeps popping up, here's an indepth explanation. It helps if the reader
-stares at print_context_stack() and the whole machinery in and around
-arch/x86/kernel/dumpstack.c.
-
-Adapted from Ingo's mail, Message-ID: <20150521101614.GA10889@gmail.com>:
-
-We always scan the full kernel stack for return addresses stored on
-the kernel stack(s) [1]_, from stack top to stack bottom, and print out
-anything that 'looks like' a kernel text address.
-
-If it fits into the frame pointer chain, we print it without a question
-mark, knowing that it's part of the real backtrace.
-
-If the address does not fit into our expected frame pointer chain we
-still print it, but we print a '?'. It can mean two things:
-
- - either the address is not part of the call chain: it's just stale
-   values on the kernel stack, from earlier function calls. This is
-   the common case.
-
- - or it is part of the call chain, but the frame pointer was not set
-   up properly within the function, so we don't recognize it.
-
-This way we will always print out the real call chain (plus a few more
-entries), regardless of whether the frame pointer was set up correctly
-or not - but in most cases we'll get the call chain right as well. The
-entries printed are strictly in stack order, so you can deduce more
-information from that as well.
-
-The most important property of this method is that we _never_ lose
-information: we always strive to print _all_ addresses on the stack(s)
-that look like kernel text addresses, so if debug information is wrong,
-we still print out the real call chain as well - just with more question
-marks than ideal.
-
-.. [1] For things like IRQ and IST stacks, we also scan those stacks, in
-       the right order, and try to cross from one stack into another
-       reconstructing the call chain. This works most of the time.