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

[Peter Zijlstra]

On Tue, Mar 10, 2026 at 09:21:22AM +0100, Uros Bizjak wrote:
> 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. As such, the "memory" clobber is unnecessary.

The point isn't that this accesses memory, but that prior or later
accesses would end up at different memory locations (as would happen
when setting the per-cpu segment.

Anyway, aside from that nit, yes these 3 patches look good to me.

> 
> No functional change intended.
> 
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> 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>
> Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> ---
>  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
>