[PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

[Denys Vlasenko <dvlasenk@redhat.com>]

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@redhat.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---

Patch was run-tested.

 arch/x86/vdso/vdso32/syscall.S | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/vdso/vdso32/syscall.S b/arch/x86/vdso/vdso32/syscall.S
index 5415b56..ccdb9ef 100644
--- a/arch/x86/vdso/vdso32/syscall.S
+++ b/arch/x86/vdso/vdso32/syscall.S
@@ -19,8 +19,15 @@ __kernel_vsyscall:
 .Lpush_ebp:
 	movl	%ecx, %ebp
 	syscall
-	movl	$__USER32_DS, %ecx
-	movl	%ecx, %ss
+	/*
+	 * Used to load __USER32_DS to %ss here,
+	 * but it's not necessary: this vDSO is only used if our kernel
+	 * is 64-bit one (and we are on AMD CPU).
+	 * For 64-bit kernels, __USER32_DS and __USER_DS are the same.
+	 * SYSRET restores %ss to the same value when returning to
+	 * either 64- or 32-bit userspace, and 64-bit kernel uses the same
+	 * descriptor for %ss in 64- and 32-bit userspace.
+	 */
 	movl	%ebp, %ecx
 	popl	%ebp
 .Lpop_ebp:
-- 
1.8.1.4