Commit d9576c9c authored by Uros Bizjak's avatar Uros Bizjak Committed by Ingo Molnar
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x86/asm/fsgsbase: Remove unnecessary "memory" clobbers from FS/GS base (read-) accessors 



The rdfsbase() and rdgsbase() helpers currently include a "memory"
clobber in their inline assembly definitions. However, the RDFSBASE
and RDGSBASE instructions only read the FS/GS base MSRs into a
general-purpose register and do not access memory. The "memory" clobber,
which acts as a compiler barrier and may inhibit optimization,
is therefore unnecessary.

The "memory" clobber was historically used as a scheduling constraint
to prevent the compiler from moving the instructions before preceding
segment register loads. This is not required because both the segment
register loads and the RDFSBASE/RDGSBASE accessors are implemented
with `asm volatile`, which already prevents reordering between them.

No functional change intended.

Signed-off-by: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://patch.msgid.link/20260330055823.5793-1-ubizjak@gmail.com
parent 7aaa8047

arch/x86/include/asm/fsgsbase.h

+2 −2
Original line number Diff line number Diff line
@@ -25,7 +25,7 @@ static __always_inline unsigned long rdfsbase(void) 
{

	unsigned long fsbase;



	asm volatile("rdfsbase %0" : "=r" (fsbase) :: "memory");

	asm volatile("rdfsbase %0" : "=r" (fsbase));



	return fsbase;

}
@@ -34,7 +34,7 @@ static __always_inline unsigned long rdgsbase(void) 
{

	unsigned long gsbase;



	asm volatile("rdgsbase %0" : "=r" (gsbase) :: "memory");

	asm volatile("rdgsbase %0" : "=r" (gsbase));



	return gsbase;

}