Re: [PATCH 4/4 v2] KVM: x86: Add a new VM statistic to show number of VCPUs created in a given VM

[Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>]

On 5/20/21 8:04 AM, Sean Christopherson wrote:
> On Wed, May 19, 2021, Krish Sadhukhan wrote:
> > 'struct kvm' already has a member for counting the number of VCPUs created
> > for a given VM. Add this as a new VM statistic to KVM debugfs.
> Huh!??  Why?  Userspace is the one creating the vCPUs, it darn well should know
> how many it's created.
>   


If I am providing a host for users to create VMs, how do I know who 
creates how many VCPUs ? This statistic is intended show usage of VCPU 
resources on a host used by customers.

> > Signed-off-by: Krish Sadhukhan <Krish.Sadhukhan@xxxxxxxxxx>
> > ---
> >   arch/x86/include/asm/kvm_host.h | 1 +
> >   arch/x86/kvm/svm/svm.c          | 3 +--
> >   arch/x86/kvm/x86.c              | 1 +
> >   virt/kvm/kvm_main.c             | 2 ++
> >   4 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index a19fe2cfaa93..69ca1d6f6557 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1139,6 +1139,7 @@ struct kvm_vm_stat {
> >   	ulong nx_lpage_splits;
> >   	ulong max_mmu_page_hash_collisions;
> >   	ulong vcpus_ran_nested;
> > +	u64 created_vcpus;
> >   };
> >   
> >   struct kvm_vcpu_stat {
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index d1871c51411f..fef0baba043b 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -3875,8 +3875,7 @@ 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_ERR) {
> ???


Sorry, this one sneaked in here from the other patch!  Will remove it.

> >   			if (!vcpu->stat.nested_runs)
> >   				++vcpu->kvm->stat.vcpus_ran_nested;
> >                           ++vcpu->stat.nested_runs;
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index cbca3609a152..a9d27ce4cc93 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -258,6 +258,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
> >   	VM_STAT("nx_largepages_splitted", nx_lpage_splits, .mode = 0444),
> >   	VM_STAT("max_mmu_page_hash_collisions", max_mmu_page_hash_collisions),
> >   	VM_STAT("vcpus_ran_nested", vcpus_ran_nested),
> > +	VM_STAT("created_vcpus", created_vcpus),
> >   	{ NULL }
> >   };
> >   
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 6b4feb92dc79..ac8f02d8a051 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -3318,6 +3318,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
> >   	}
> >   
> >   	kvm->created_vcpus++;
> > +	kvm->stat.created_vcpus++;
> >   	mutex_unlock(&kvm->lock);
> >   
> >   	r = kvm_arch_vcpu_precreate(kvm, id);
> > @@ -3394,6 +3395,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
> >   vcpu_decrement:
> >   	mutex_lock(&kvm->lock);
> >   	kvm->created_vcpus--;
> > +	kvm->stat.created_vcpus--;
> >   	mutex_unlock(&kvm->lock);
> >   	return r;
> >   }
> > --
> > 2.27.0