[PATCH -tip v3 1/4] x86/fsgsbase: Remove unnecessary "memory" clobbers from FS/GS base accessors

[Uros Bizjak]

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: Uros Bizjak <ubizjak@xxxxxxxxx>
Acked-by: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
---
v2: Note that the "memory" clobber was historically
    used as a scheduling constraint.
---
 arch/x86/include/asm/fsgsbase.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
index ab2547f97c2c..70ff4ef457b1 100644
--- a/arch/x86/include/asm/fsgsbase.h
+++ b/arch/x86/include/asm/fsgsbase.h
@@ -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;
 }
-- 
2.53.0