x86/asm/segment: Remove unnecessary "memory" clobber from savesegment()

The savesegment() macro uses inline assembly to copy a segment register
into a general-purpose register:

  movl %seg, reg

This instruction does not access memory, yet the inline asm currently
declares a "memory" clobber, which unnecessarily acts as a compiler
barrier and may inhibit optimization.

Remove the "memory" clobber and mark the asm as `asm volatile` instead.
Segment register loads in the kernel are implemented using `asm volatile`,
so the compiler will not schedule segment register reads before those
loads. Using `asm volatile` preserves the intended ordering with other
segment register operations without imposing an unnecessary global memory
barrier.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://patch.msgid.link/20260330055823.5793-2-ubizjak@gmail.com

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 9f5be2bbd2918994b3998678d1a01a99b57fb17b..3fe3a310844c4681ed3ef7fefe540e16ee8496c9 100644 (file)
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -348,7 +348,7 @@ static inline void __loadsegment_fs(unsigned short value)
  * Save a segment register away:
  */
 #define savesegment(seg, value)                                \
-       asm("movl %%" #seg ",%k0" : "=r" (value) : : "memory")
+       asm volatile("movl %%" #seg ",%k0" : "=r" (value))
 
 #endif /* !__ASSEMBLER__ */
 #endif /* __KERNEL__ */