Re: [PATCH] KVM: LAPIC: Reset timer_advance_ns if timer mode switch

[Wanpeng Li]

On Mon, 31 Aug 2020 at 20:48, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> Wanpeng Li <kernellwp@xxxxxxxxx> writes:
>
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > per-vCPU timer_advance_ns should be set to 0 if timer mode is not tscdeadline
> > otherwise we waste cpu cycles in the function lapic_timer_int_injected(),
>
> lapic_timer_int_injected is just a test, kvm_wait_lapic_expire()
> (__kvm_wait_lapic_expire()) maybe?

Both the check in lapic_timer_int_injected(), the check in
__kvm_wait_lapic_expire(), and these function calls, we can observe
~1.3% world switch time reduce w/ this patch by
kvm-unit-tests/vmexit.flat vmcall testing on AMD server. In addition,
I think we should set apic->lapic_timer.expired_tscdeadline to 0 when
switching between tscdeadline mode and other modes on Intel in order
that we will not waste cpu cycles to tune advance value in
adjust_lapic_timer_advance() for one time.

Wanpeng