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[PATCH] x86: document sysenter path 

This path isn't obvious.  It looks as if the kernel will be taking three
args from the user stack, but it only takes one from there.

Signed-off-by: Albert Cahalan <acahalan@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Albert D. Cahalan 2006-02-14 13:53:20 -08:00 committed by Linus Torvalds
parent 28baebae73
commit 581141cb4b
1 changed files with 15 additions and 0 deletions
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15
arch/i386/kernel/vsyscall-sysenter.S
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@ -7,6 +7,21 @@
* for details.
*/
/*
* The caller puts arg2 in %ecx, which gets pushed. The kernel will use
* %ecx itself for arg2. The pushing is because the sysexit instruction
* (found in entry.S) requires that we clobber %ecx with the desired %esp.
* User code might expect that %ecx is unclobbered though, as it would be
* for returning via the iret instruction, so we must push and pop.
*
* The caller puts arg3 in %edx, which the sysexit instruction requires
* for %eip. Thus, exactly as for arg2, we must push and pop.
*
* Arg6 is different. The caller puts arg6 in %ebp. Since the sysenter
* instruction clobbers %esp, the user's %esp won't even survive entry
* into the kernel. We store %esp in %ebp. Code in entry.S must fetch
* arg6 from the stack.
*/
.text
.globl __kernel_vsyscall
.type __kernel_vsyscall,@function