2017-06-28 6:09 GMT+08:00 Wanpeng Li <kernellwp@xxxxxxxxx>:
> 2017-06-28 0:10 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>>
>>
>> On 23/06/2017 13:59, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>>
>>> 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
