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

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/4/2025 2:45 AM, dongli.zhang@oracle.com wrote:
> 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.

I just sent a series for CPUID_EXT_PDCM[1]. I think you can put 
CPUID_EXT3_PERFCORE and CPUID_8000_0022_EAX_PERFMON_V2 at the same place.

[1] 
https://lore.kernel.org/qemu-devel/20250304052450.465445-1-xiaoyao.li@intel.com/T/#m31c6777131b6361d7c3af22b09532bdc785dbc06

> 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:
>>
>