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Based on kernel version 4.1. Page generated on 2015-06-28 12:15 EST.

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