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

[Sean Christopherson <seanjc@google.com>]

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.
 
> Signed-off-by: Krish Sadhukhan <Krish.Sadhukhan@oracle.com>
> ---
>  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) {

???

>  			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
>