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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:05 EST.

1	This file documents some of the kernel entries in
2	arch/x86/kernel/entry_64.S.  A lot of this explanation is adapted from
3	an email from Ingo Molnar:
4	
5	http://lkml.kernel.org/r/<20110529191055.GC9835%40elte.hu>
6	
7	The x86 architecture has quite a few different ways to jump into
8	kernel code.  Most of these entry points are registered in
9	arch/x86/kernel/traps.c and implemented in arch/x86/kernel/entry_64.S
10	and arch/x86/ia32/ia32entry.S.
11	
12	The IDT vector assignments are listed in arch/x86/include/irq_vectors.h.
13	
14	Some of these entries are:
15	
16	 - system_call: syscall instruction from 64-bit code.
17	
18	 - ia32_syscall: int 0x80 from 32-bit or 64-bit code; compat syscall
19	   either way.
20	
21	 - ia32_syscall, ia32_sysenter: syscall and sysenter from 32-bit
22	   code
23	
24	 - interrupt: An array of entries.  Every IDT vector that doesn't
25	   explicitly point somewhere else gets set to the corresponding
26	   value in interrupts.  These point to a whole array of
27	   magically-generated functions that make their way to do_IRQ with
28	   the interrupt number as a parameter.
29	
30	 - APIC interrupts: Various special-purpose interrupts for things
31	   like TLB shootdown.
32	
33	 - Architecturally-defined exceptions like divide_error.
34	
35	There are a few complexities here.  The different x86-64 entries
36	have different calling conventions.  The syscall and sysenter
37	instructions have their own peculiar calling conventions.  Some of
38	the IDT entries push an error code onto the stack; others don't.
39	IDT entries using the IST alternative stack mechanism need their own
40	magic to get the stack frames right.  (You can find some
41	documentation in the AMD APM, Volume 2, Chapter 8 and the Intel SDM,
42	Volume 3, Chapter 6.)
43	
44	Dealing with the swapgs instruction is especially tricky.  Swapgs
45	toggles whether gs is the kernel gs or the user gs.  The swapgs
46	instruction is rather fragile: it must nest perfectly and only in
47	single depth, it should only be used if entering from user mode to
48	kernel mode and then when returning to user-space, and precisely
49	so. If we mess that up even slightly, we crash.
50	
51	So when we have a secondary entry, already in kernel mode, we *must
52	not* use SWAPGS blindly - nor must we forget doing a SWAPGS when it's
53	not switched/swapped yet.
54	
55	Now, there's a secondary complication: there's a cheap way to test
56	which mode the CPU is in and an expensive way.
57	
58	The cheap way is to pick this info off the entry frame on the kernel
59	stack, from the CS of the ptregs area of the kernel stack:
60	
61		xorl %ebx,%ebx
62		testl $3,CS+8(%rsp)
63		je error_kernelspace
64		SWAPGS
65	
66	The expensive (paranoid) way is to read back the MSR_GS_BASE value
67	(which is what SWAPGS modifies):
68	
69		movl $1,%ebx
70		movl $MSR_GS_BASE,%ecx
71		rdmsr
72		testl %edx,%edx
73		js 1f   /* negative -> in kernel */
74		SWAPGS
75		xorl %ebx,%ebx
76	1:	ret
77	
78	and the whole paranoid non-paranoid macro complexity is about whether
79	to suffer that RDMSR cost.
80	
81	If we are at an interrupt or user-trap/gate-alike boundary then we can
82	use the faster check: the stack will be a reliable indicator of
83	whether SWAPGS was already done: if we see that we are a secondary
84	entry interrupting kernel mode execution, then we know that the GS
85	base has already been switched. If it says that we interrupted
86	user-space execution then we must do the SWAPGS.
87	
88	But if we are in an NMI/MCE/DEBUG/whatever super-atomic entry context,
89	which might have triggered right after a normal entry wrote CS to the
90	stack but before we executed SWAPGS, then the only safe way to check
91	for GS is the slower method: the RDMSR.
92	
93	So we try only to mark those entry methods 'paranoid' that absolutely
94	need the more expensive check for the GS base - and we generate all
95	'normal' entry points with the regular (faster) entry macros.
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