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x86/vdso32/syscall.S: Do not load __USER32_DS to %ss 

This vDSO code only gets used by 64-bit kernels, not 32-bit ones.

On 64-bit kernels, the data segment is the same for 32-bit and
64-bit userspace, and the SYSRET instruction loads %ss with its
selector.

So there's no need to repeat it by hand. Segment loads are somewhat
expensive: tens of cycles.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
[ Removed unnecessary comment. ]
Signed-off-by: Andy Lutomirski <luto@kernel.org>
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/63da6d778f69fd0f1345d9287f6764d58be519fa.1427482099.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Denys Vlasenko 2015-03-27 11:48:17 -07:00 committed by Ingo Molnar
parent 0a4f59d6e0
commit e7d6eefaaa
1 changed files with 0 additions and 2 deletions
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arch/x86/vdso/vdso32/syscall.S
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@ -19,8 +19,6 @@ __kernel_vsyscall:
.Lpush_ebp:
movl %ecx, %ebp
syscall
movl $__USER32_DS, %ecx
movl %ecx, %ss
movl %ebp, %ecx
popl %ebp
.Lpop_ebp: