Re: [RFC PATCH v1 1/4] x86/cpufeatures: Add CPUID feature bit for the Bus Lock Threshold

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 09, 2024, Manali Shukla wrote:
> Malicious guests can cause bus locks to degrade the performance of

I would say "misbehaving", I bet the overwhelming majority of bus locks in practice
are due to legacy/crusty software, not malicious software.

> a system. Non-WB(write-back) and misaligned locked
> RMW(read-modify-write) instructions are referred to as "bus locks" and
> require system wide synchronization among all processors to guarantee
> atomicity.  The bus locks may incur significant performance penalties
> for all processors in the system.
> 
> The Bus Lock Threshold feature proves beneficial for hypervisors
> seeking to restrict guests' ability to initiate numerous bus locks,
> thereby preventing system slowdowns that affect all tenants.

None of this actually says what the feature does.

> Presence of the Bus Lock threshold feature is indicated via CPUID
> function 0x8000000A_EDX[29]
> 
> Signed-off-by: Manali Shukla <manali.shukla@amd.com>
> ---
>  arch/x86/include/asm/cpufeatures.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 3c7434329661..10f397873790 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -381,6 +381,7 @@
>  #define X86_FEATURE_V_SPEC_CTRL		(15*32+20) /* Virtual SPEC_CTRL */
>  #define X86_FEATURE_VNMI		(15*32+25) /* Virtual NMI */
>  #define X86_FEATURE_SVME_ADDR_CHK	(15*32+28) /* "" SVME addr check */
> +#define X86_FEATURE_BUS_LOCK_THRESHOLD	(15*32+29) /* "" Bus lock threshold */

I would strongly prefer to enumerate this in /proc/cpuinfo, having to manually
query CPUID to see if a CPU supports a feature I want to test is beyond annoying.

>  /* Intel-defined CPU features, CPUID level 0x00000007:0 (ECX), word 16 */
>  #define X86_FEATURE_AVX512VBMI		(16*32+ 1) /* AVX512 Vector Bit Manipulation instructions*/
> 
> base-commit: 704ec48fc2fbd4e41ec982662ad5bf1eee33eeb2
> -- 
> 2.34.1
>