Re: [PATCH 04/10] KVM: x86: Set PVCLOCK_GUEST_STOPPED only for kvmclock, not for Xen PV clock

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 21, 2025, Paul Durrant wrote:
> On 21/01/2025 17:09, Sean Christopherson wrote:
> > On Tue, Jan 21, 2025, Paul Durrant wrote:
> > > > ---
> > > >    arch/x86/kvm/x86.c | 20 ++++++++++++++------
> > > >    1 file changed, 14 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index d8ee37dd2b57..3c4d210e8a9e 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -3150,11 +3150,6 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v,
> > > >    	/* retain PVCLOCK_GUEST_STOPPED if set in guest copy */
> > > >    	vcpu->hv_clock.flags |= (guest_hv_clock->flags & PVCLOCK_GUEST_STOPPED);
> > > > -	if (vcpu->pvclock_set_guest_stopped_request) {
> > > > -		vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED;
> > > > -		vcpu->pvclock_set_guest_stopped_request = false;
> > > > -	}
> > > > -
> > > >    	memcpy(guest_hv_clock, &vcpu->hv_clock, sizeof(*guest_hv_clock));
> > > >    	if (force_tsc_unstable)
> > > > @@ -3264,8 +3259,21 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> > > >    	if (use_master_clock)
> > > >    		vcpu->hv_clock.flags |= PVCLOCK_TSC_STABLE_BIT;
> > > > -	if (vcpu->pv_time.active)
> > > > +	if (vcpu->pv_time.active) {
> > > > +		/*
> > > > +		 * GUEST_STOPPED is only supported by kvmclock, and KVM's
> > > > +		 * historic behavior is to only process the request if kvmclock
> > > > +		 * is active/enabled.
> > > > +		 */
> > > > +		if (vcpu->pvclock_set_guest_stopped_request) {
> > > > +			vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED;
> > > > +			vcpu->pvclock_set_guest_stopped_request = false;
> > > > +		}
> > > >    		kvm_setup_guest_pvclock(v, &vcpu->pv_time, 0, false);
> > > > +
> > > > +		vcpu->hv_clock.flags &= ~PVCLOCK_GUEST_STOPPED;
> > > 
> > > Is this intentional? The line above your change in kvm_setup_guest_pvclock()
> > > clearly keeps the flag enabled if it already set and, without this patch, I
> > > don't see anything clearing it.
> > 
> > Oh, I see what you're getting at.  Hrm.  Yes, clearing the flag is intentional,
> > otherwise the patch wouldn't do what it claims to do (set PVCLOCK_GUEST_STOPPED
> > only for kvmclock).
> > 
> > Swapping the order of this patch and the next patch ("don't bleed ...") doesn't
> > break the cycle because that would result in PVCLOCK_GUEST_STOPPED only being
> > applied to the first active clock (kvmclock).
> > 
> > The only way I can think of to fully isolate the changes would be to split this
> > into two patches: (4a) hoist pvclock_set_guest_stopped_request processing into
> > kvm_guest_time_update() and (4b) apply it only to kvmclock, and then make the
> > ordering 4a, 5, 4b, i.e. "hoist", "don't bleed", "only kvmclock".
> > 
> > 4a would be quite ugly, because to avoid introducing a functional change, it
> > would need to be:
> > 
> > 	if (vcpu->pv_time.active || vcpu->xen.vcpu_info_cache.active ||
> > 	    vcpu->xen.vcpu_time_info_cache.active) {
> > 		vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED;
> > 		vcpu->pvclock_set_guest_stopped_request = false;
> > 	}
> > 
> > But it's not the worst intermediate code, so I'm not opposed to going that
> > route.
> > 
> 
> What about putting this change after patch 7. Then you could take a local
> copy of hv_clock in which you could set PVCLOCK_GUEST_STOPPED and so avoid
> bleeding the flag that way?

But to preserve the current behavior of setting PVCLOCK_GUEST_STOPPED for all
clocks, processing pvclock_set_guest_stopped_request needs to be moved out of
kvm_setup_guest_pvclock() before said helper can make a copy of the reference.