linux-kernel - Re: [PATCH] KVM: LAPIC: Fix lapic timer injection delay

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Date:   Fri, 23 Jun 2017 17:25:55 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] KVM: LAPIC: Fix lapic timer injection delay

2017-06-23 17:22 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> From: Wanpeng Li <wanpeng.li@...mail.com>
>
> Linux as a guest will prevent to program the next event to the clock event
> device when the event is in the past. However, it is not guaranteed by all
> the guests, the guest like kvm-unit-tests/apic.flat will write current tsc
> value visible in guest to MSR_IA32_TSCDEADLINE, lapic timer which is emulated
> by vmx preemption timer will program the absolute target tsc value to vmcs
> preemption timer field w/ delta == 0, then plays a vmentry and an upcoming
> vmx preemption timer fire vmexit dance, the lapic timer injection is delayed
> for this duration. Actually the lapic timer which is emulated by hrtimer can
> handle this correctly.
>
> This patch fixes it by firing the lapic timer and injecting a timer interrupt
> immediately during the next vmentry if guest programs an expired timer to
> the emulated timer device.
>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kvm/lapic.c | 6 +++++-
>  arch/x86/kvm/vmx.c   | 3 +++
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index d24c874..8a1263a 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1504,12 +1504,16 @@ static void cancel_hv_timer(struct kvm_lapic *apic)
>  static bool start_hv_timer(struct kvm_lapic *apic)
>  {
>         u64 tscdeadline = apic->lapic_timer.tscdeadline;
> +       int ret = 0;
>
>         if ((atomic_read(&apic->lapic_timer.pending) &&
>                 !apic_lvtt_period(apic)) ||
> -               kvm_x86_ops->set_hv_timer(apic->vcpu, tscdeadline)) {
> +               (ret = kvm_x86_ops->set_hv_timer(apic->vcpu, tscdeadline))) {
>                 if (apic->lapic_timer.hv_timer_in_use)
>                         cancel_hv_timer(apic);
> +               if (ret == 1)
> +                       apic_timer_expired(apic);
> +               return true;

Ouch! I just sent a wrong version.

Regards,
Wanpeng Li

>         } else {
>                 apic->lapic_timer.hv_timer_in_use = true;
>                 hrtimer_cancel(&apic->lapic_timer.timer);
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 2e906cf..2008e9b 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -11149,6 +11149,9 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
>         u64 guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
>         u64 delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
>
> +       if (delta_tsc == 0)
> +               return 1;
> +
>         /* Convert to host delta tsc if tsc scaling is enabled */
>         if (vcpu->arch.tsc_scaling_ratio != kvm_default_tsc_scaling_ratio &&
>                         u64_shl_div_u64(delta_tsc,
> --
> 2.7.4
>