linux-kernel - Re: [PATCH 3/3] KVM: LAPIC: Fix lapic timer injection delay

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Date:   Sun, 2 Jul 2017 09:35:29 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Radim Krcmar <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH 3/3] KVM: LAPIC: Fix lapic timer injection delay

2017-06-30 0:58 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
> From: Wanpeng Li <wanpeng.li@...mail.com>
>
> If the TSC deadline timer is programmed really close to the deadline or
> even in the past, the computation in vmx_set_hv_timer will program the
> absolute target tsc value to vmcs preemption timer field w/ delta == 0.
> The next vmentry results in an immediate vmx preemption timer vmexit
> and the lapic timer injection is delayed due to 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 the TSC deadline timer is programmed
> really close to the deadline or even in the past. This saves ~1200 cycles on
> the tscdeadline_immed test of vmexit.flat.
>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> [Rebased on top of previous patch. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/lapic.c | 5 ++++-
>  arch/x86/kvm/vmx.c   | 3 ++-
>  2 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index a80e5a5d6f2f..2819d4c123eb 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1525,8 +1525,11 @@ static bool start_hv_timer(struct kvm_lapic *apic)
>          * the window.  For periodic timer, leave the hv timer running for
>          * simplicity, and the deadline will be recomputed on the next vmexit.
>          */
> -       if (!apic_lvtt_period(apic) && atomic_read(&ktimer->pending))
> +       if (!apic_lvtt_period(apic) && (r || atomic_read(&ktimer->pending))) {
> +               if (r)
> +                       apic_timer_expired(apic);
>                 return false;
> +       }

This logic is not the same as in my v4
http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1434040.html
. You return false for the expired timer and actually it will switch
to sw timer.

>
>         trace_kvm_hv_timer_state(apic->vcpu->vcpu_id, true);
>         return true;
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index e8b61ad84a8e..92ddea08f999 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -11147,7 +11147,8 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
>         vmx->hv_deadline_tsc = tscl + delta_tsc;
>         vmcs_set_bits(PIN_BASED_VM_EXEC_CONTROL,
>                         PIN_BASED_VMX_PREEMPTION_TIMER);
> -       return 0;
> +
> +       return delta_tsc == 0;
>  }
>
>  static void vmx_cancel_hv_timer(struct kvm_vcpu *vcpu)
> --
> 1.8.3.1
>