Re: [PATCH v2 02/10] target/i386: disable PERFCORE when "-pmu" is configured

[dongli.zhang@oracle.com]

Hi Xiaoyao,

On 3/2/25 5:59 PM, Xiaoyao Li wrote:
> On 3/3/2025 6:00 AM, Dongli Zhang wrote:
>> Currently, AMD PMU support isn't determined based on CPUID, that is, the
>> "-pmu" option does not fully disable KVM AMD PMU virtualization.
>>
>> To minimize AMD PMU features, remove PERFCORE when "-pmu" is configured.
>>
>> To completely disable AMD PMU virtualization will be implemented via
>> KVM_CAP_PMU_CAPABILITY in upcoming patches.
>>
>> As a reminder, neither CPUID_EXT3_PERFCORE nor
>> CPUID_8000_0022_EAX_PERFMON_V2 is removed from env->features[] when "-pmu"
>> is configured. Developers should query whether they are supported via
>> cpu_x86_cpuid() rather than relying on env->features[] in future patches.
> 
> I don't think it is the correct direction to go.
> 
> env->features[] should be finalized before cpu_x86_cpuid() and env-
>>features[] needs to be able to be exposed to guest directly. This ensures
> guest and QEMU have the same view of CPUIDs and it simplifies things.
> 
> We can adjust env->features[] by filtering all PMU related CPUIDs based on
> cpu->enable_pmu in x86_cpu_realizefn().

Thank you very much for suggestion.

I see  code like below in x86_cpu_realizefn() to edit env->features[].

7982     /* On AMD CPUs, some CPUID[8000_0001].EDX bits must match the bits on
7983      * CPUID[1].EDX.
7984      */
7985     if (IS_AMD_CPU(env)) {
7986         env->features[FEAT_8000_0001_EDX] &= ~CPUID_EXT2_AMD_ALIASES;
7987         env->features[FEAT_8000_0001_EDX] |= (env->features[FEAT_1_EDX]
7988            & CPUID_EXT2_AMD_ALIASES);
7989     }

I may do something similar to them for CPUID_EXT3_PERFCORE and
CPUID_8000_0022_EAX_PERFMON_V2.

Dongli Zhang



> 
>> Suggested-by: Zhao Liu <zhao1.liu@intel.com>
>> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
>> ---
>>   target/i386/cpu.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index b6d6167910..61a671028a 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -7115,6 +7115,10 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t
>> index, uint32_t count,
>>               !(env->hflags & HF_LMA_MASK)) {
>>               *edx &= ~CPUID_EXT2_SYSCALL;
>>           }
>> +
>> +        if (kvm_enabled() && IS_AMD_CPU(env) && !cpu->enable_pmu) {
>> +            *ecx &= ~CPUID_EXT3_PERFCORE;
>> +        }
>>           break;
>>       case 0x80000002:
>>       case 0x80000003:
>