linux/Documentation/x86/x86_64/kernel-stacks
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   1Most of the text from Keith Owens, hacked by AK
   2
   3x86_64 page size (PAGE_SIZE) is 4K.
   4
   5Like all other architectures, x86_64 has a kernel stack for every
   6active thread.  These thread stacks are THREAD_SIZE (2*PAGE_SIZE) big.
   7These stacks contain useful data as long as a thread is alive or a
   8zombie. While the thread is in user space the kernel stack is empty
   9except for the thread_info structure at the bottom.
  10
  11In addition to the per thread stacks, there are specialized stacks
  12associated with each CPU.  These stacks are only used while the kernel
  13is in control on that CPU; when a CPU returns to user space the
  14specialized stacks contain no useful data.  The main CPU stacks are:
  15
  16* Interrupt stack.  IRQSTACKSIZE
  17
  18  Used for external hardware interrupts.  If this is the first external
  19  hardware interrupt (i.e. not a nested hardware interrupt) then the
  20  kernel switches from the current task to the interrupt stack.  Like
  21  the split thread and interrupt stacks on i386, this gives more room
  22  for kernel interrupt processing without having to increase the size
  23  of every per thread stack.
  24
  25  The interrupt stack is also used when processing a softirq.
  26
  27Switching to the kernel interrupt stack is done by software based on a
  28per CPU interrupt nest counter. This is needed because x86-64 "IST"
  29hardware stacks cannot nest without races.
  30
  31x86_64 also has a feature which is not available on i386, the ability
  32to automatically switch to a new stack for designated events such as
  33double fault or NMI, which makes it easier to handle these unusual
  34events on x86_64.  This feature is called the Interrupt Stack Table
  35(IST).  There can be up to 7 IST entries per CPU. The IST code is an
  36index into the Task State Segment (TSS). The IST entries in the TSS
  37point to dedicated stacks; each stack can be a different size.
  38
  39An IST is selected by a non-zero value in the IST field of an
  40interrupt-gate descriptor.  When an interrupt occurs and the hardware
  41loads such a descriptor, the hardware automatically sets the new stack
  42pointer based on the IST value, then invokes the interrupt handler.  If
  43software wants to allow nested IST interrupts then the handler must
  44adjust the IST values on entry to and exit from the interrupt handler.
  45(This is occasionally done, e.g. for debug exceptions.)
  46
  47Events with different IST codes (i.e. with different stacks) can be
  48nested.  For example, a debug interrupt can safely be interrupted by an
  49NMI.  arch/x86_64/kernel/entry.S::paranoidentry adjusts the stack
  50pointers on entry to and exit from all IST events, in theory allowing
  51IST events with the same code to be nested.  However in most cases, the
  52stack size allocated to an IST assumes no nesting for the same code.
  53If that assumption is ever broken then the stacks will become corrupt.
  54
  55The currently assigned IST stacks are :-
  56
  57* STACKFAULT_STACK.  EXCEPTION_STKSZ (PAGE_SIZE).
  58
  59  Used for interrupt 12 - Stack Fault Exception (#SS).
  60
  61  This allows the CPU to recover from invalid stack segments. Rarely
  62  happens.
  63
  64* DOUBLEFAULT_STACK.  EXCEPTION_STKSZ (PAGE_SIZE).
  65
  66  Used for interrupt 8 - Double Fault Exception (#DF).
  67
  68  Invoked when handling one exception causes another exception. Happens
  69  when the kernel is very confused (e.g. kernel stack pointer corrupt).
  70  Using a separate stack allows the kernel to recover from it well enough
  71  in many cases to still output an oops.
  72
  73* NMI_STACK.  EXCEPTION_STKSZ (PAGE_SIZE).
  74
  75  Used for non-maskable interrupts (NMI).
  76
  77  NMI can be delivered at any time, including when the kernel is in the
  78  middle of switching stacks.  Using IST for NMI events avoids making
  79  assumptions about the previous state of the kernel stack.
  80
  81* DEBUG_STACK.  DEBUG_STKSZ
  82
  83  Used for hardware debug interrupts (interrupt 1) and for software
  84  debug interrupts (INT3).
  85
  86  When debugging a kernel, debug interrupts (both hardware and
  87  software) can occur at any time.  Using IST for these interrupts
  88  avoids making assumptions about the previous state of the kernel
  89  stack.
  90
  91* MCE_STACK.  EXCEPTION_STKSZ (PAGE_SIZE).
  92
  93  Used for interrupt 18 - Machine Check Exception (#MC).
  94
  95  MCE can be delivered at any time, including when the kernel is in the
  96  middle of switching stacks.  Using IST for MCE events avoids making
  97  assumptions about the previous state of the kernel stack.
  98
  99For more details see the Intel IA32 or AMD AMD64 architecture manuals.
 100
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