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

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Date:   Wed, 28 Jun 2017 06:09:39 +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 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):

 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