[tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS to %ss

[tip-bot for Denys Vlasenko]

Commit-ID:  e7d6eefaaa443130079d73cd05039d90b3db7a4a
Gitweb:     http://git.kernel.org/tip/e7d6eefaaa443130079d73cd05039d90b3db7a4a
Author:     Denys Vlasenko <dvlasenk@xxxxxxxxxx>
AuthorDate: Fri, 27 Mar 2015 11:48:17 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 31 Mar 2015 10:45:15 +0200

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@xxxxxxxxxx>
[ Removed unnecessary comment. ]
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/63da6d778f69fd0f1345d9287f6764d58be519fa.1427482099.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/vdso/vdso32/syscall.S | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/vdso/vdso32/syscall.S b/arch/x86/vdso/vdso32/syscall.S
index 5415b56..6b286bb 100644
--- a/arch/x86/vdso/vdso32/syscall.S
+++ b/arch/x86/vdso/vdso32/syscall.S
@@ -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:
--
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