[PATCH -tip 1/4] x86/elf: Use savesegment() for segment register reads in ELF core dump

[Uros Bizjak <ubizjak@gmail.com>]

ELF_CORE_COPY_REGS() currently reads %ds, %es, %fs, and %gs using
inline assembly and manual zero-extension. This results in redundant
instructions like `mov %eax,%eax`.

Replace the inline assembly with the `savesegment()` helper, which
automatically zero-extends the value to the full register width,
eliminating unnecessary instructions.

For example, the %ds load sequence changes from:

  d03: 8c d8                 mov    %ds,%eax
  d05: 89 c0                 mov    %eax,%eax
  d07: 48 89 84 24 38 01 00  mov    %rax,0x138(%rsp)
  d0e: 00

to:

  ce8: 8c d8                 mov    %ds,%eax
  cea: 48 89 84 24 38 01 00  mov    %rax,0x138(%rsp)
  cf1: 00

thus eliminating the unnecessary zero-extending `mov %eax,%eax`.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
 arch/x86/include/asm/elf.h | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 2ba5f166e58f..c7f98977663c 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -187,7 +187,6 @@ void set_personality_ia32(bool);
 
 #define ELF_CORE_COPY_REGS(pr_reg, regs)			\
 do {								\
-	unsigned v;						\
 	(pr_reg)[0] = (regs)->r15;				\
 	(pr_reg)[1] = (regs)->r14;				\
 	(pr_reg)[2] = (regs)->r13;				\
@@ -211,10 +210,10 @@ do {								\
 	(pr_reg)[20] = (regs)->ss;				\
 	(pr_reg)[21] = x86_fsbase_read_cpu();			\
 	(pr_reg)[22] = x86_gsbase_read_cpu_inactive();		\
-	asm("movl %%ds,%0" : "=r" (v)); (pr_reg)[23] = v;	\
-	asm("movl %%es,%0" : "=r" (v)); (pr_reg)[24] = v;	\
-	asm("movl %%fs,%0" : "=r" (v)); (pr_reg)[25] = v;	\
-	asm("movl %%gs,%0" : "=r" (v)); (pr_reg)[26] = v;	\
+	savesegment(ds, (pr_reg)[23]);				\
+	savesegment(es, (pr_reg)[24]);				\
+	savesegment(fs, (pr_reg)[25]);				\
+	savesegment(gs, (pr_reg)[26]);				\
 } while (0);
 
 /* I'm not sure if we can use '-' here */
-- 
2.53.0