linux-kernel - Re: [PATCH v2] KVM: LAPIC: Fix lapic timer injection delay

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Date:   Wed, 28 Jun 2017 06:10:30 +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 Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2] KVM: LAPIC: Fix lapic timer injection delay

2017-06-28 6:09 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> 2017-06-28 0:10 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>>
>>
>> On 23/06/2017 13:59, Wanpeng Li wrote:
>>> 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. This saves ~300 cycles on the tsc_deadline_timer
>>> test of apic.flat.
>>
>> If it doesn't matter for actual guests, I'd rather not complicate the logic.
>
> Sorry, maybe my description is not that accurate. It can also benefit
> if the tsc deadline timer is programmed really close to the deadline.
> That's why we introduce commit 9175d2e97b ("KVM: vmx: fix underflow in
> TSC deadline calculation"). In addition, I capture there is frequently
> programmed close to the deadline in vmx_set_hv_timer(delta_tsc == 0):

I test a linux guest.

>
>  qemu-system-x86-2352  [003] ....   457.709435: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [004] ....   457.941090: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.381340: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.381565: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.382079: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.382277: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.382576: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.383065: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.565506: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.565718: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.566199: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.566431: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.566680: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [005] ....   458.567154: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [006] ....   458.989358: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [006] ....   458.989554: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [006] ....   458.990075: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [003] ....   459.397456: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [003] ....   459.397618: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [003] ....   459.397877: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [003] ....   459.398091: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [003] ....   459.398307: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [003] ....   459.398672: vmx_set_hv_timer: expired timer
>  qemu-system-x86-2352  [003] ....   459.502826: vmx_set_hv_timer: expired timer
>
> Regards,
> Wanpeng Li