Re: [PATCH v3 1/4] x86/cpufeatures: Add CPUID feature bit for the Bus Lock Threshold

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

On 10/6/2024 11:56 AM, Borislav Petkov wrote:
> On October 4, 2024 7:33:38 AM GMT+02:00, Manali Shukla <manali.shukla@amd.com> wrote:
>> Misbehaving guests can cause bus locks to degrade the performance of
>> the system. The Bus Lock Threshold feature can be used to address this
>> issue by providing capability to the hypervisor to limit guest's
>> ability to generate bus lock, thereby preventing system slowdown due
>> to performance penalities.
>>
>> When the Bus Lock Threshold feature is enabled, the processor checks
>> the bus lock threshold count before executing the buslock and decides
>> whether to trigger bus lock exit or not.
>>
>> The value of the bus lock threshold count '0' generates bus lock
>> exits, and if the value is greater than '0', the bus lock is executed
>> successfully and the bus lock threshold count is decremented.
>>
>> 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 dd4682857c12..77fa8e743ccc 100644
>> --- a/arch/x86/include/asm/cpufeatures.h
>> +++ b/arch/x86/include/asm/cpufeatures.h
>> @@ -382,6 +382,7 @@
>> #define X86_FEATURE_V_SPEC_CTRL		(15*32+20) /* "v_spec_ctrl" Virtual SPEC_CTRL */
>> #define X86_FEATURE_VNMI		(15*32+25) /* "vnmi" Virtual NMI */
>> #define X86_FEATURE_SVME_ADDR_CHK	(15*32+28) /* SVME addr check */
>> +#define X86_FEATURE_BUS_LOCK_THRESHOLD	(15*32+29) /* "buslock" Bus lock threshold */
> 
> Why does this feature flag need to be visible in /proc/cpuinfo? 
> 
> 
This was already discussed on [v1] of the same series but never concluded.
As suggested in the discussion [v1], I have added "buslock" to be enumerated
in "/proc/cpuinfo". I really don't have any strong opinions about adding or
removing it from "/proc/cpuinfo". So, I would let maintainers decide how to
go about it.

v1: https://lore.kernel.org/kvm/76355a11-a0ba-4a28-bf51-454facfd59e5@amd.com/T/#mb225b5b5fad2c070d4fadd51a076e9a05330b6a3

-Manali