Re: [PATCH] x86/barrier: Do not serialize MSR accesses on AMD

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 22, 2023 at 11:52:12AM +0200, Borislav Petkov wrote:
> From: "Borislav Petkov (AMD)" <bp@alien8.de>
> 
> AMD does not have the requirement for a synchronization barrier when
> acccessing a certain group of MSRs. Do not incur that unnecessary
> penalty there.

So you're saying that AMD tsc_deadline and x2apic MSRs *do* imply
ordering constraints unlike the Intel ones?

Can we pls haz a document link for that, also a comment?

> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
> ---
>  arch/x86/include/asm/barrier.h   | 18 ------------------
>  arch/x86/include/asm/processor.h | 19 +++++++++++++++++++
>  2 files changed, 19 insertions(+), 18 deletions(-)

Moving this code while changing it meant I had to look at it _3_ times
before I spotted you changed it :/

> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index b216ac80ebcc..983406342484 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -735,4 +735,23 @@ bool arch_is_platform_page(u64 paddr);
>  #define arch_is_platform_page arch_is_platform_page
>  #endif
>  
> +/*
> + * Make previous memory operations globally visible before
> + * a WRMSR.
> + *
> + * MFENCE makes writes visible, but only affects load/store
> + * instructions.  WRMSR is unfortunately not a load/store
> + * instruction and is unaffected by MFENCE.  The LFENCE ensures
> + * that the WRMSR is not reordered.
> + *
> + * Most WRMSRs are full serializing instructions themselves and
> + * do not require this barrier.  This is only required for the
> + * IA32_TSC_DEADLINE and X2APIC MSRs.
> + */
> +static inline void weak_wrmsr_fence(void)
> +{
> +	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
> +		asm volatile("mfence; lfence" : : : "memory");

Both instructions are 3 bytes, a 6 byte nop would be better, no?

	asm volatile (ALTERNATIVE("mfence; lfence;", "", X86_FEATURE_AMD));

or something ?

> +}
> +
>  #endif /* _ASM_X86_PROCESSOR_H */
> -- 
> 2.35.1
>