Re: [PATCH 3/4 v2] KVM: nVMX: nSVM: Add a new debugfs statistic to show how many VCPUs have run nested guests

[Krish Sadhukhan <krish.sadhukhan@oracle.com>]

On 5/20/21 9:57 AM, Jim Mattson wrote:
> On Thu, May 20, 2021 at 7:56 AM Sean Christopherson <seanjc@google.com> wrote:
>> On Wed, May 19, 2021, Krish Sadhukhan wrote:
>>> Add a new debugfs statistic to show how many VCPUs have run nested guests.
>>> This statistic considers only the first time a given VCPU successfully runs
>>> a nested guest.
>>>
>>> Signed-off-by: Krish Sadhukhan <Krish.Sadhukhan@oracle.com>
>>> Suggested-by: Jim Mattson <jmattson@google.com>
>>> ---
>>>   arch/x86/include/asm/kvm_host.h | 1 +
>>>   arch/x86/kvm/svm/svm.c          | 5 ++++-
>>>   arch/x86/kvm/vmx/vmx.c          | 5 ++++-
>>>   arch/x86/kvm/x86.c              | 1 +
>>>   4 files changed, 10 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>> index cf8557b2b90f..a19fe2cfaa93 100644
>>> --- a/arch/x86/include/asm/kvm_host.h
>>> +++ b/arch/x86/include/asm/kvm_host.h
>>> @@ -1138,6 +1138,7 @@ struct kvm_vm_stat {
>>>        ulong lpages;
>>>        ulong nx_lpage_splits;
>>>        ulong max_mmu_page_hash_collisions;
>>> +     ulong vcpus_ran_nested;
>>>   };
>>>
>>>   struct kvm_vcpu_stat {
>>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>>> index 57c351640355..d1871c51411f 100644
>>> --- a/arch/x86/kvm/svm/svm.c
>>> +++ b/arch/x86/kvm/svm/svm.c
>>> @@ -3876,8 +3876,11 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>>>                /* Track VMRUNs that have made past consistency checking */
>>>                if (svm->nested.nested_run_pending &&
>>>                    svm->vmcb->control.exit_code != SVM_EXIT_ERR &&
>>> -                 svm->vmcb->control.exit_code != SVM_EXIT_NPF)
>>> +                 svm->vmcb->control.exit_code != SVM_EXIT_NPF) {
>>> +                     if (!vcpu->stat.nested_runs)
>>> +                             ++vcpu->kvm->stat.vcpus_ran_nested;
>> Using a separate counter seems unnecessary, userspace can aggregate
>> vcpu->stat.nested_run itself to see how many vCPUs have done nested VM-Enter.
>>
>> Jim, were you thinking of something else?  Am I missing something?
> It was in the context of a proposed stat to indicate how many vCPUs
> are *currently* running nested guests that I said I'd rather just know
> how many vCPUs had *ever* run nested guests. I don't need a separate
> stat. Checking vcpu->stat.nested_run for non-zero values works fine
> for me.
I will fall back to my v1 idea then. That's at least useful if we want 
to create a time graph of VCPUs running nested guests.