linux-kernel - Re: [PATCH] KVM: LAPIC: Advancing the timer expiration on guest initiated write

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Date:   Wed, 3 Mar 2021 08:12:43 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Advancing the timer expiration on guest
 initiated write

On Wed, Mar 03, 2021, Wanpeng Li wrote:
> > Side topic, I think there's a theoretical bug where KVM could inject a spurious
> > timer interrupt.  If KVM is using hrtimer, the hrtimer expires early due to an
> > overzealous timer_advance_ns, and the guest writes MSR_TSCDEADLINE after the
> > hrtimer expires but before the vCPU is kicked, then KVM will inject a spurious
> > timer IRQ since the premature expiration should have been canceled by the guest's
> > WRMSR.
> >
> > It could also cause KVM to soft hang the guest if the new lapic_timer.tscdeadline
> > is written before apic_timer_expired() captures it in expired_tscdeadline.  In
> > that case, KVM will wait for the new deadline, which could be far in the future.
> 
> The hrtimer_cancel() before setting new lapic_timer.tscdeadline in
> kvm_set_lapic_tscdeadline_msr() will wait for the hrtimer callback
> function to finish. Could it solve this issue?

Aha!  Yep, that prevents my theoretical bug.  Thanks!