From: Denys Vlasenko Date: Fri, 15 May 2015 20:39:06 +0000 (+0200) Subject: x86/asm/entry/64: Use shorter MOVs from segment registers X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=adeb5537849d9db428fe0ddc3562e5a765a347e2;p=openwrt%2Fstaging%2Fblogic.git x86/asm/entry/64: Use shorter MOVs from segment registers The "movw %ds,%cx" instruction needs a 0x66 prefix, while "movl %ds,%ecx" does not. The difference is that latter form (on 64-bit CPUs) overwrites the entire %ecx, not only its lower half. But subsequent code doesn't depend on the value of upper half of %ecx, so we can safely use the shorter instruction. The new code is also faster than the old one - now we don't depend on the old value of %ecx, but this code fragment is not performance-critical so it does not matter much. Signed-off-by: Denys Vlasenko Cc: Alexei Starovoitov Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Frederic Weisbecker Cc: H. Peter Anvin Cc: Kees Cook Cc: Linus Torvalds Cc: Oleg Nesterov Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Thomas Gleixner Cc: Will Drewry Link: http://lkml.kernel.org/r/1431722346-26585-1-git-send-email-dvlasenk@redhat.com Signed-off-by: Ingo Molnar --- diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 09c3f9e0e07e..47b95813dc37 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -1190,17 +1190,17 @@ 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). */