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

[Manali Shukla <manali.shukla@amd.com>]

Hi Sean,

Thank you for reviewing my patches.

On 8/17/2024 1:07 AM, Sean Christopherson wrote:
> 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.
> 

Ack.

>> 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.
> 

Sure I will rewrite the commit message. 

>> 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.

I will do the modifications accordingly.

> 
>>  /* 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
>>
 - Manali