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x86/asm/entry/64: Improve the THREAD_INFO() macro explanation

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Explain the background, and add a real example.

Acked-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/20150324184311.GA14760@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Ingo Molnar 2015-03-24 19:43:11 +01:00
parent d56fe4bf5f
commit 1ddc6f3c60
1 changed files with 23 additions and 4 deletions
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27
arch/x86/include/asm/thread_info.h
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@ -206,10 +206,29 @@ static inline unsigned long current_stack_pointer(void)
_ASM_SUB $(THREAD_SIZE),reg ;
/*
* ASM operand which evaluates to thread_info address
* if it is known that "reg" is exactly "off" bytes below stack top.
* Example (fetch thread_info->fieldname):
* mov TI_fieldname+THREAD_INFO(reg, off),%eax
* ASM operand which evaluates to a 'thread_info' address of
* the current task, if it is known that "reg" is exactly "off"
* bytes below the top of the stack currently.
*
* ( The kernel stack's size is known at build time, it is usually
* 2 or 4 pages, and the bottom of the kernel stack contains
* the thread_info structure. So to access the thread_info very
* quickly from assembly code we can calculate down from the
* top of the kernel stack to the bottom, using constant,
* build-time calculations only. )
*
* For example, to fetch the current thread_info->flags value into %eax
* on x86-64 defconfig kernels, in syscall entry code where RSP is
* currently at exactly SIZEOF_PTREGS bytes away from the top of the
* stack:
*
* mov TI_flags+THREAD_INFO(%rsp, SIZEOF_PTREGS), %eax
*
* will translate to:
*
* 8b 84 24 b8 c0 ff ff mov -0x3f48(%rsp), %eax
*
* which is below the current RSP by almost 16K.
*/
#define THREAD_INFO(reg, off) ((off)-THREAD_SIZE)(reg)