linux-kernel - Re: [PATCH 2/2] KVM: x86: Identify vCPU0 by its vcpu

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Message-ID: <87czpd2bsi.fsf@vitty.brq.redhat.com>
Date:   Mon, 13 Sep 2021 09:07:57 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Identify vCPU0 by its vcpu_idx instead of
 walking vCPUs array

Sean Christopherson <seanjc@...gle.com> writes:

> Use vcpu_idx to identify vCPU0 when updating HyperV's TSC page, which is
> shared by all vCPUs and "owned" by vCPU0 (because vCPU0 is the only vCPU
> that's guaranteed to exist).  Using kvm_get_vcpu() to find vCPU works,
> but it's a rather odd and suboptimal method to check the index of a given
> vCPU.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 86539c1686fa..6ab851df08d1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2969,7 +2969,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>  				       offsetof(struct compat_vcpu_info, time));
>  	if (vcpu->xen.vcpu_time_info_set)
>  		kvm_setup_pvclock_page(v, &vcpu->xen.vcpu_time_info_cache, 0);
> -	if (v == kvm_get_vcpu(v->kvm, 0))
> +	if (!v->vcpu_idx)
>  		kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock);
>  	return 0;
>  }

" ... instead of walking vCPUs array" in the Subject line is a bit
confusing because kvm_get_vcpu() doesn't actually walk anything, it just
returns 'kvm->vcpus[i]' after checking that we actually have that many
vCPUs. The patch itself is OK, so

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly