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

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Date:   Tue, 9 Aug 2022 14:07:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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
Subject: Re: [PATCH v3 2/2] KVM: x86/xen: Stop Xen timer before changing IRQ

On Tue, Aug 09, 2022, David Woodhouse wrote:
> On Tue, 2022-08-09 at 14:59 +0200, Paolo Bonzini wrote:
> > On 8/9/22 11:22, David Woodhouse wrote:
> > > On Mon, 2022-08-08 at 14:06 -0500, Coleman Dietsch wrote:
> > > > Stop Xen timer (if it's running) prior to changing the IRQ vector and
> > > > potentially (re)starting the timer. Changing the IRQ vector while the
> > > > timer is still running can result in KVM injecting a garbage event, e.g.
> > > > vm_xen_inject_timer_irqs() could see a non-zero xen.timer_pending from
> > > > a previous timer but inject the new xen.timer_virq.
> > > 
> > > Hm, wasn't that already addressed in the first patch I saw, which just
> > > called kvm_xen_stop_timer() unconditionally before (possibly) setting
> > > it up again?
> > 
> > Which patch is that?
> 
> The one I acked in
> https://lore.kernel.org/all/9bad724858b6a06c25ead865b2b3d9dfc216d01c.camel@infradead.org/

It's effectively the same patch.  I had asked Coleman to split it into two separate
patches: (1) fix the re-initialization of an active timer bug and (2) stop the active
timer before changing the vector (this patch).