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Linux/Documentation/arch/x86/kernel-stacks.rst

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Diff markup

Differences between /Documentation/arch/x86/kernel-stacks.rst (Architecture m68k) and /Documentation/arch/i386/kernel-stacks.rst (Architecture i386)


  1 .. SPDX-License-Identifier: GPL-2.0               
  2                                                   
  3 =============                                     
  4 Kernel Stacks                                     
  5 =============                                     
  6                                                   
  7 Kernel stacks on x86-64 bit                       
  8 ===========================                       
  9                                                   
 10 Most of the text from Keith Owens, hacked by A    
 11                                                   
 12 x86_64 page size (PAGE_SIZE) is 4K.               
 13                                                   
 14 Like all other architectures, x86_64 has a ker    
 15 active thread.  These thread stacks are THREAD    
 16 These stacks contain useful data as long as a     
 17 zombie. While the thread is in user space the     
 18 except for the thread_info structure at the bo    
 19                                                   
 20 In addition to the per thread stacks, there ar    
 21 associated with each CPU.  These stacks are on    
 22 is in control on that CPU; when a CPU returns     
 23 specialized stacks contain no useful data.  Th    
 24                                                   
 25 * Interrupt stack.  IRQ_STACK_SIZE                
 26                                                   
 27   Used for external hardware interrupts.  If t    
 28   hardware interrupt (i.e. not a nested hardwa    
 29   kernel switches from the current task to the    
 30   the split thread and interrupt stacks on i38    
 31   for kernel interrupt processing without havi    
 32   of every per thread stack.                      
 33                                                   
 34   The interrupt stack is also used when proces    
 35                                                   
 36 Switching to the kernel interrupt stack is don    
 37 per CPU interrupt nest counter. This is needed    
 38 hardware stacks cannot nest without races.        
 39                                                   
 40 x86_64 also has a feature which is not availab    
 41 to automatically switch to a new stack for des    
 42 double fault or NMI, which makes it easier to     
 43 events on x86_64.  This feature is called the     
 44 (IST).  There can be up to 7 IST entries per C    
 45 index into the Task State Segment (TSS). The I    
 46 point to dedicated stacks; each stack can be a    
 47                                                   
 48 An IST is selected by a non-zero value in the     
 49 interrupt-gate descriptor.  When an interrupt     
 50 loads such a descriptor, the hardware automati    
 51 pointer based on the IST value, then invokes t    
 52 the interrupt came from user mode, then the in    
 53 will switch back to the per-thread stack.  If     
 54 nested IST interrupts then the handler must ad    
 55 entry to and exit from the interrupt handler.     
 56 done, e.g. for debug exceptions.)                 
 57                                                   
 58 Events with different IST codes (i.e. with dif    
 59 nested.  For example, a debug interrupt can sa    
 60 NMI.  arch/x86_64/kernel/entry.S::paranoidentr    
 61 pointers on entry to and exit from all IST eve    
 62 IST events with the same code to be nested.  H    
 63 stack size allocated to an IST assumes no nest    
 64 If that assumption is ever broken then the sta    
 65                                                   
 66 The currently assigned IST stacks are:            
 67                                                   
 68 * ESTACK_DF.  EXCEPTION_STKSZ (PAGE_SIZE).        
 69                                                   
 70   Used for interrupt 8 - Double Fault Exceptio    
 71                                                   
 72   Invoked when handling one exception causes a    
 73   when the kernel is very confused (e.g. kerne    
 74   Using a separate stack allows the kernel to     
 75   in many cases to still output an oops.          
 76                                                   
 77 * ESTACK_NMI.  EXCEPTION_STKSZ (PAGE_SIZE).       
 78                                                   
 79   Used for non-maskable interrupts (NMI).         
 80                                                   
 81   NMI can be delivered at any time, including     
 82   middle of switching stacks.  Using IST for N    
 83   assumptions about the previous state of the     
 84                                                   
 85 * ESTACK_DB.  EXCEPTION_STKSZ (PAGE_SIZE).        
 86                                                   
 87   Used for hardware debug interrupts (interrup    
 88   debug interrupts (INT3).                        
 89                                                   
 90   When debugging a kernel, debug interrupts (b    
 91   software) can occur at any time.  Using IST     
 92   avoids making assumptions about the previous    
 93   stack.                                          
 94                                                   
 95   To handle nested #DB correctly there exist t    
 96   #DB entry the IST stackpointer for #DB is sw    
 97   so a nested #DB starts from a clean stack. T    
 98   the IST stackpointer to a guard hole to catc    
 99                                                   
100 * ESTACK_MCE.  EXCEPTION_STKSZ (PAGE_SIZE).       
101                                                   
102   Used for interrupt 18 - Machine Check Except    
103                                                   
104   MCE can be delivered at any time, including     
105   middle of switching stacks.  Using IST for M    
106   assumptions about the previous state of the     
107                                                   
108 For more details see the Intel IA32 or AMD AMD    
109                                                   
110                                                   
111 Printing backtraces on x86                        
112 ==========================                        
113                                                   
114 The question about the '?' preceding function     
115 keeps popping up, here's an indepth explanatio    
116 stares at print_context_stack() and the whole     
117 arch/x86/kernel/dumpstack.c.                      
118                                                   
119 Adapted from Ingo's mail, Message-ID: <20150521    
120                                                   
121 We always scan the full kernel stack for retur    
122 the kernel stack(s) [1]_, from stack top to st    
123 anything that 'looks like' a kernel text addre    
124                                                   
125 If it fits into the frame pointer chain, we pr    
126 mark, knowing that it's part of the real backt    
127                                                   
128 If the address does not fit into our expected     
129 still print it, but we print a '?'. It can mea    
130                                                   
131  - either the address is not part of the call     
132    values on the kernel stack, from earlier fu    
133    the common case.                               
134                                                   
135  - or it is part of the call chain, but the fr    
136    up properly within the function, so we don'    
137                                                   
138 This way we will always print out the real cal    
139 entries), regardless of whether the frame poin    
140 or not - but in most cases we'll get the call     
141 entries printed are strictly in stack order, s    
142 information from that as well.                    
143                                                   
144 The most important property of this method is     
145 information: we always strive to print _all_ a    
146 that look like kernel text addresses, so if de    
147 we still print out the real call chain as well    
148 marks than ideal.                                 
149                                                   
150 .. [1] For things like IRQ and IST stacks, we     
151        the right order, and try to cross from     
152        reconstructing the call chain. This wor    
                                                      

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