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Diffstat (limited to 'Documentation/x86/kernel-stacks')
| -rw-r--r-- | Documentation/x86/kernel-stacks | 141 |
1 files changed, 0 insertions, 141 deletions
diff --git a/Documentation/x86/kernel-stacks b/Documentation/x86/kernel-stacks deleted file mode 100644 index 9a0aa4d3a866..000000000000 --- a/Documentation/x86/kernel-stacks +++ /dev/null @@ -1,141 +0,0 @@ -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 :- - -* DOUBLEFAULT_STACK. 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. - -* NMI_STACK. 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. - -* DEBUG_STACK. DEBUG_STKSZ - - 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. - -* MCE_STACK. 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) [*], 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. - -[*] 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. |