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

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Message-ID: <Z4_U16jb7IbVdlLi@google.com>
Date: Tue, 21 Jan 2025 09:09:43 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: paul@....org
Cc: Paolo Bonzini <pbonzini@...hat.com>, David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	syzbot+352e553a86e0d75f5120@...kaller.appspotmail.com, 
	Paul Durrant <pdurrant@...zon.com>, David Woodhouse <dwmw@...zon.co.uk>, 
	Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 04/10] KVM: x86: Set PVCLOCK_GUEST_STOPPED only for
 kvmclock, not for Xen PV clock

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.

> > +	}
> > +
> >   #ifdef CONFIG_KVM_XEN
> >   	if (vcpu->xen.vcpu_info_cache.active)
> >   		kvm_setup_guest_pvclock(v, &vcpu->xen.vcpu_info_cache,
>