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

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Date:   Mon, 20 Sep 2021 14:57:15 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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

On Mon, Sep 13, 2021, Vitaly Kuznetsov wrote:
> 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

Argh, yes, I have a feeling I wrote the changelog after digging into the history
of kvm_get_vcpu().

Paolo, can you tweak the shortlog to:

  KVM: x86: Identify vCPU0 by its vcpu_idx instead of its vCPUs array entry