Re: [PATCH 2/2] KVM: x86: Identify vCPU0 by its vcpu_idx instead of walking vCPUs array

[Paolo Bonzini]

On 20/09/21 16:57, Sean Christopherson wrote:
> On Mon, Sep 13, 2021, Vitaly Kuznetsov wrote:
> > Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >   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

Done and queued.  Patch 1 required some further s390 changes.

Paolo