[PATCH v3] x86/asm/entry/64: Use shorter MOVs from segment registers

[Denys Vlasenko <dvlasenk@redhat.com>]

"movw %ds,%cx" insn needs a 0x66 prefix, while "movl %ds,%ecx" does not.
The difference is that latter form (on 64-bit CPUs) overwrites
entire %ecx, not only its lower half.

But subsequent code doesn't depend on the value of upper
half of %ecx, we can safely use the shorter insn.

The new code is also faster than old one - now we don't depend on old value
of %ecx, but this code fragment is not performance-critical.

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
---
Changes in v2: clarified that 32-bit form overwrites entire %ecx
*on 64-bit CPUs*. Rumor has it old 486s/P5s exist which don't do that,
or they write garbage in upper half.

Changes in v2: fixed typos in commit message

 arch/x86/kernel/entry_64.S | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 62b4c5f..ef2651d 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1195,19 +1195,19 @@ ENTRY(xen_failsafe_callback)
 	/*CFI_REL_OFFSET ds,DS*/
 	CFI_REL_OFFSET r11,8
 	CFI_REL_OFFSET rcx,0
-	movw %ds,%cx
+	movl %ds,%ecx
 	cmpw %cx,0x10(%rsp)
 	CFI_REMEMBER_STATE
 	jne 1f
-	movw %es,%cx
+	movl %es,%ecx
 	cmpw %cx,0x18(%rsp)
 	jne 1f
-	movw %fs,%cx
+	movl %fs,%ecx
 	cmpw %cx,0x20(%rsp)
 	jne 1f
-	movw %gs,%cx
+	movl %gs,%ecx
 	cmpw %cx,0x28(%rsp)
 	jne 1f
 	/* All segments match their saved values => Category 2 (Bad IRET). */
 	movq (%rsp),%rcx
 	CFI_RESTORE rcx
 	movq 8(%rsp),%r11
-- 
1.8.1.4