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Peter Anvin" Subject: [PATCH -tip v3 1/4] x86/fsgsbase: Remove unnecessary "memory" clobbers from FS/GS base accessors Date: Thu, 12 Mar 2026 22:58:55 +0100 Message-ID: <20260312215916.258176-1-ubizjak@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 Acked-by: "Peter Zijlstra (Intel)" Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: Dave Hansen Cc: "H. Peter Anvin" --- 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