linux-kernel - Re: [PATCH v3 2/2] KVM: x86/xen: Stop Xen timer before changing IRQ

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Message-ID: <YvJx3Dje4zS/c+H0@google.com>
Date:   Tue, 9 Aug 2022 14:40:28 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Coleman Dietsch <dietschc@....edu>, kvm@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        skhan@...uxfoundation.org, Pavel Skripkin <paskripkin@...il.com>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        stable@...r.kernel.org,
        syzbot+e54f930ed78eb0f85281@...kaller.appspotmail.com,
        metikaya <metikaya@...zon.co.uk>
Subject: Re: [PATCH v3 2/2] KVM: x86/xen: Stop Xen timer before changing IRQ

On Tue, Aug 09, 2022, Paolo Bonzini wrote:
> On 8/9/22 16:16, David Woodhouse wrote:
> > I find the new version a bit harder to follow, with its init-then-stop-
> > then-start logic:
> > 
> > 	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> > 		if (data->u.timer.port &&
> > 		    data->u.timer.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) {
> > 			r = -EINVAL;
> > 			break;
> >                  }
> > 
> > 		if (!vcpu->arch.xen.timer.function)
> > 			kvm_xen_init_timer(vcpu);
> > 
> > 		/* Stop the timer (if it's running) before changing the vector */
> > 		kvm_xen_stop_timer(vcpu);
> > 		vcpu->arch.xen.timer_virq = data->u.timer.port;
> 
> 
> I think this is fine, if anything the kvm_xen_stop_timer() call could be
> placed in an "else" but I'm leaning towards applying this version of the
> patch.

I wanted to separate the "init" from the "stop+start", e.g. if there were a more
appropriate place for invoking kvm_xen_init_timer() I would have suggested moving
the call outside of KVM_XEN_VCPU_ATTR_TYPE_TIMER entirely.