[GIT PULL 2/4] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

[Andy Lutomirski]

From: Denys Vlasenko <dvlasenk@xxxxxxxxxx>

This vDSO code only gets used by 64-bit kernel,
not 32-bit. In 64-bit kernels, data segment is the same
for 32-bit and 64-bit userspace, and SYSRET insn does load %ss
with its selector. No need to repeat it by hand. Segment loads
are somewhat expensive: tens of cycles.

Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Borislav Petkov <bp@xxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
CC: Oleg Nesterov <oleg@xxxxxxxxxx>
CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
CC: Alexei Starovoitov <ast@xxxxxxxxxxxx>
CC: Will Drewry <wad@xxxxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: x86@xxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
Message-Id: <1427129240-15543-1-git-send-email-dvlasenk@xxxxxxxxxx>
[removed unnecessary comment]
Signed-off-by: Andy Lutomirski <luto@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 5415b5613d55..6b286bb5251c 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:
-- 
2.3.0

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