Re: [PATCH 5/5] KVM: x86: Using TSC deadline may cause multiple interrupts by user writes

[Paolo Bonzini <pbonzini@redhat.com>]

Il 08/10/2014 12:06, Radim Krčmář ha scritto:
> KVM: x86: fix deadline tsc interrupt injection
> 
> The check in kvm_set_lapic_tscdeadline_msr() was trying to prevent a
> situation where we lose a pending deadline timer in a MSR write.
> Losing it is fine, because it effectively occurs before the timer fired,
> so we should be able to cancel or postpone it.
> 
> Another problem comes from interaction with QEMU, or other userspace
> that can set deadline MSR without a good reason, when timer is already
> pending:  one guest's deadline request results in more than one
> interrupt because one is injected immediately on MSR write from
> userspace and one through hrtimer later.
> 
> The solution is to remove the injection when replacing a pending timer
> and to improve the usual QEMU path, we inject without a hrtimer when the
> deadline has already passed.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> Reported-by: Nadav Amit <namit@cs.technion.ac.il>
> ---
>  arch/x86/kvm/lapic.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index b8345dd..51428dd 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1096,9 +1096,12 @@ static void start_apic_timer(struct kvm_lapic *apic)
>  		if (likely(tscdeadline > guest_tsc)) {
>  			ns = (tscdeadline - guest_tsc) * 1000000ULL;
>  			do_div(ns, this_tsc_khz);
> +			hrtimer_start(&apic->lapic_timer.timer,
> +				ktime_add_ns(now, ns), HRTIMER_MODE_ABS);
> +		} else {
> +			atomic_inc(&ktimer->pending);
> +			kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
>  		}
> -		hrtimer_start(&apic->lapic_timer.timer,
> -			ktime_add_ns(now, ns), HRTIMER_MODE_ABS);
>  
>  		local_irq_restore(flags);
>  	}
> @@ -1355,9 +1358,6 @@ void kvm_set_lapic_tscdeadline_msr(struct kvm_vcpu *vcpu, u64 data)
>  		return;
>  
>  	hrtimer_cancel(&apic->lapic_timer.timer);
> -	/* Inject here so clearing tscdeadline won't override new value */
> -	if (apic_has_pending_timer(vcpu))
> -		kvm_inject_apic_timer_irqs(vcpu);
>  	apic->lapic_timer.tscdeadline = data;
>  	start_apic_timer(apic);
>  }

Radim, the patch looks good but please extract this code:

        /*
         * There is a race window between reading and incrementing, but we do
         * not care about potentially losing timer events in the !reinject
         * case anyway. Note: KVM_REQ_PENDING_TIMER is implicitly checked
         * in vcpu_enter_guest.
         */
        if (!atomic_read(&ktimer->pending)) {
                atomic_inc(&ktimer->pending);
                /* FIXME: this code should not know anything about vcpus */
                kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
        }

        if (waitqueue_active(q))
                wake_up_interruptible(q);

to a new "static void apic_timer_expired(struct kvm_lapic *apic)" function,
and call it from both apic_timer_fn and start_apic_timer.

Also, we should not need to do wake_up_interruptible() unless we have
changed ktimer->pending from zero to non-zero.

Paolo