Re: [PATCH 1/2] KVM: LAPIC: write 0 to TMICT should also cancel vmx-preemption timer

From: David Matlack <dmatlack@google.com>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: linux-kernel@vger.kernel.org, kvm list <kvm@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH 1/2] KVM: LAPIC: write 0 to TMICT should also cancel vmx-preemption timer
Date: Thu, 3 Jun 2021 16:02:23 -0700	[thread overview]
Message-ID: <CALzav=e9pbkk0=Yz9s1b+53MEy7yuo_otoFM75fNeoJGCQjqCg@mail.gmail.com> (raw)
In-Reply-To: <1622710841-76604-1-git-send-email-wanpengli@tencent.com>

On Thu, Jun 3, 2021 at 2:04 AM Wanpeng Li <kernellwp@gmail.com> wrote:
>
> From: Wanpeng Li <wanpengli@tencent.com>
>
> According to the SDM 10.5.4.1:
>
>   A write of 0 to the initial-count register effectively stops the local
>   APIC timer, in both one-shot and periodic mode.

If KVM is not correctly emulating this behavior then could you also
add a kvm-unit-test to test for the correct behavior?

>
> The lapic timer oneshot/periodic mode which is emulated by vmx-preemption
> timer doesn't stop since vmx->hv_deadline_tsc is still set. This patch
> fixes it by also cancel vmx-preemption timer when writing 0 to initial-count
> register.
>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/lapic.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 8120e86..20dd2ae 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1494,6 +1494,15 @@ static void limit_periodic_timer_frequency(struct kvm_lapic *apic)
>
>  static void cancel_hv_timer(struct kvm_lapic *apic);
>
> +static void cancel_timer(struct kvm_lapic *apic)
> +{
> +       hrtimer_cancel(&apic->lapic_timer.timer);
> +       preempt_disable();
> +       if (apic->lapic_timer.hv_timer_in_use)
> +               cancel_hv_timer(apic);
> +       preempt_enable();
> +}
> +
>  static void apic_update_lvtt(struct kvm_lapic *apic)
>  {
>         u32 timer_mode = kvm_lapic_get_reg(apic, APIC_LVTT) &
> @@ -1502,11 +1511,7 @@ static void apic_update_lvtt(struct kvm_lapic *apic)
>         if (apic->lapic_timer.timer_mode != timer_mode) {
>                 if (apic_lvtt_tscdeadline(apic) != (timer_mode ==
>                                 APIC_LVT_TIMER_TSCDEADLINE)) {
> -                       hrtimer_cancel(&apic->lapic_timer.timer);
> -                       preempt_disable();
> -                       if (apic->lapic_timer.hv_timer_in_use)
> -                               cancel_hv_timer(apic);
> -                       preempt_enable();
> +                       cancel_timer(apic);
>                         kvm_lapic_set_reg(apic, APIC_TMICT, 0);
>                         apic->lapic_timer.period = 0;
>                         apic->lapic_timer.tscdeadline = 0;
> @@ -2092,7 +2097,7 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
>                 if (apic_lvtt_tscdeadline(apic))
>                         break;
>
> -               hrtimer_cancel(&apic->lapic_timer.timer);
> +               cancel_timer(apic);
>                 kvm_lapic_set_reg(apic, APIC_TMICT, val);
>                 start_apic_timer(apic);
>                 break;
> --
> 2.7.4
>