Re: [PATCH 2/3] i386: kvm: disable KVM_CAP_PMU_CAPABILITY if "pmu" is disabled

[Dongli Zhang]

Hi Greg and Liang,

On 11/21/22 6:23 AM, Liang Yan wrote:
> 
> On 11/21/22 06:03, Greg Kurz wrote:
>> On Sat, 19 Nov 2022 04:29:00 -0800
>> Dongli Zhang <dongli.zhang@oracle.com> wrote:
>>
>>> The "perf stat" at the VM side still works even we set "-cpu host,-pmu" in
>>> the QEMU command line. That is, neither "-cpu host,-pmu" nor "-cpu EPYC"
>>> could disable the pmu virtualization in an AMD environment.
>>>
>>> We still see below at VM kernel side ...
>>>
>>> [    0.510611] Performance Events: Fam17h+ core perfctr, AMD PMU driver.
>>>
>>> ... although we expect something like below.
>>>
>>> [    0.596381] Performance Events: PMU not available due to virtualization,
>>> using software events only.
>>> [    0.600972] NMI watchdog: Perf NMI watchdog permanently disabled
>>>
>>> This is because the AMD pmu (v1) does not rely on cpuid to decide if the
>>> pmu virtualization is supported.
>>>
>>> We disable KVM_CAP_PMU_CAPABILITY if the 'pmu' is disabled in the vcpu
>>> properties.
>>>
>>> Cc: Joe Jin <joe.jin@oracle.com>
>>> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
>>> ---
>>>   target/i386/kvm/kvm.c | 17 +++++++++++++++++
>>>   1 file changed, 17 insertions(+)
>>>
>>> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
>>> index 8fec0bc5b5..0b1226ff7f 100644
>>> --- a/target/i386/kvm/kvm.c
>>> +++ b/target/i386/kvm/kvm.c
>>> @@ -137,6 +137,8 @@ static int has_triple_fault_event;
>>>     static bool has_msr_mcg_ext_ctl;
>>>   +static int has_pmu_cap;
>>> +
>>>   static struct kvm_cpuid2 *cpuid_cache;
>>>   static struct kvm_cpuid2 *hv_cpuid_cache;
>>>   static struct kvm_msr_list *kvm_feature_msrs;
>>> @@ -1725,6 +1727,19 @@ static void kvm_init_nested_state(CPUX86State *env)
>>>     void kvm_arch_pre_create_vcpu(CPUState *cs)
>>>   {
>>> +    X86CPU *cpu = X86_CPU(cs);
>>> +    int ret;
>>> +
>>> +    if (has_pmu_cap && !cpu->enable_pmu) {
>>> +        ret = kvm_vm_enable_cap(kvm_state, KVM_CAP_PMU_CAPABILITY, 0,
>>> +                                KVM_PMU_CAP_DISABLE);
>> It doesn't seem conceptually correct to configure VM level stuff out of
>> a vCPU property, which could theoretically be different for each vCPU,
>> even if this isn't the case with the current code base.
>>
>> Maybe consider controlling PMU with a machine property and this
>> could be done in kvm_arch_init() like other VM level stuff ?
>>
> 
> There is already a 'pmu' property for x86_cpu with variable 'enable_pmu' as we
> see the above code. It is mainly used by Intel CPU and set to off by default
> since qemu 1.5.
> 
> And, this property is spread to AMD CPU too.
> 
> I think you may need setup a machine property to disable it from current 
> machine
> model. Otherwise, it will break the Live Migration scenario.

Initially, I was thinking about to replace the CPU level property 'pmu' to a VM
level property but with a concern that reviewers/maintainers might not be happy
with it. That may break the way other Apps use the QEMU.

I will add an extra VM level property to the kvm_accel_class_init() for
"-machine accel=kvm" in addition to the existing per CPU property. The VM (kvm)
level
'pmu' will be more privileged than the CPU level 'pmu'.

Thank you very much!

Dongli Zhang

> 
> 
>>> +        if (ret < 0) {
>>> +            error_report("kvm: Failed to disable pmu cap: %s",
>>> +                         strerror(-ret));
>>> +        }
>>> +
>>> +        has_pmu_cap = 0;
>>> +    }
>>>   }
>>>     int kvm_arch_init_vcpu(CPUState *cs)
>>> @@ -2517,6 +2532,8 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>>>           }
>>>       }
>>>   +    has_pmu_cap = kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY);
>>> +
>>>       ret = kvm_get_supported_msrs(s);
>>>       if (ret < 0) {
>>>           return ret;
>>