x86/elf: Use savesegment() for segment register reads in ELF core dump

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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://patch.msgid.link/20260330085938.67985-1-ubizjak@gmail.com

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 2ba5f166e58fee4c2cd194daaddfe2712b845641..c7f98977663ca40e5c45b3b18bae07f700e2fad0 100644 (file)
--- 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 */