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

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Date:   Tue, 2 Mar 2021 09:16:41 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, 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 Tue, Mar 02, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Advancing the timer expiration should only be necessary on guest initiated 
> writes. Now, we cancel the timer, clear .pending and clear expired_tscdeadline 
> at the same time during state restore.

That last sentence is confusing.  kvm_apic_set_state() already clears .pending,
by way of __start_apic_timer().  I think what you mean is:

  When we cancel the timer and clear .pending during state restore, clear
  expired_tscdeadline as well.

With that, 

Reviewed-by: Sean Christopherson <seanjc@...gle.com> 

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.

Side topic #2, I'm pretty sure the direct usage of kvm_wait_lapic_expire() in
apic_timer_expired() before kvm_apic_inject_pending_timer_irqs() is broken.
kvm_wait_lapic_expire() requires the interrupt to be pending, but that never
happens if PI is used, and even if PI "fails", the IRQ isn't injected until the
next line, kvm_apic_inject_pending_timer_irqs().  I'll send a patch.

> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/lapic.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 45d40bf..f2b6e79 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2595,6 +2595,7 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
>  
>  	apic_update_ppr(apic);
>  	hrtimer_cancel(&apic->lapic_timer.timer);
> +	apic->lapic_timer.expired_tscdeadline = 0;
>  	apic_update_lvtt(apic);
>  	apic_manage_nmi_watchdog(apic, kvm_lapic_get_reg(apic, APIC_LVT0));
>  	update_divide_count(apic);
> -- 
> 2.7.4
>