[PATCH -tip v2 1/3] x86/fsgsbase: Remove unnecessary "memory" clobbers from FS/GS base accessors

[PATCH -tip v2 1/3] 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@gmail.com>
Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
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>
---
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

[PATCH -tip v2 2/3] x86/segment: Remove unnecessary memory clobber from savesegment()

[Uros Bizjak]

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>
Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
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/segment.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 9f5be2bbd291..3fe3a310844c 100644
--- 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__ */
-- 
2.53.0

[PATCH -tip v2 3/3] x86/segment: Use ASM_INPUT_RM in __loadsegment_fs()

[Uros Bizjak]

Use the ASM_INPUT_RM macro in __loadsegment_fs() to work around clang
problems with "rm" asm constraint. clang seems to always chose the
memory input, while it is almost always the worst choice.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
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>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <nick.desaulniers+lkml@gmail.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
---
 arch/x86/include/asm/segment.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 3fe3a310844c..0f4283dcd0c4 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -335,7 +335,7 @@ static inline void __loadsegment_fs(unsigned short value)
 
 		     _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_CLEAR_FS)
 
-		     : : "rm" (value) : "memory");
+		     : : ASM_INPUT_RM (value) : "memory");
 }
 
 /* __loadsegment_gs is intentionally undefined.  Use load_gs_index instead. */
-- 
2.53.0