Based on kernel version 4.0. Page generated on 2015-04-14 21:26 EST.
1 Most of the text from Keith Owens, hacked by AK 2 3 x86_64 page size (PAGE_SIZE) is 4K. 4 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. 10 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: 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 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. 30 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. 38 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.) 48 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. 56 57 The currently assigned IST stacks are :- 58 59 * STACKFAULT_STACK. EXCEPTION_STKSZ (PAGE_SIZE). 60 61 Used for interrupt 12 - Stack Fault Exception (#SS). 62 63 This allows the CPU to recover from invalid stack segments. Rarely 64 happens. 65 66 * DOUBLEFAULT_STACK. EXCEPTION_STKSZ (PAGE_SIZE). 67 68 Used for interrupt 8 - Double Fault Exception (#DF). 69 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. 74 75 * NMI_STACK. EXCEPTION_STKSZ (PAGE_SIZE). 76 77 Used for non-maskable interrupts (NMI). 78 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. 82 83 * DEBUG_STACK. DEBUG_STKSZ 84 85 Used for hardware debug interrupts (interrupt 1) and for software 86 debug interrupts (INT3). 87 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. 92 93 * MCE_STACK. EXCEPTION_STKSZ (PAGE_SIZE). 94 95 Used for interrupt 18 - Machine Check Exception (#MC). 96 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. 100 101 For more details see the Intel IA32 or AMD AMD64 architecture manuals.