linux-kernel - Re: [PATCH] KVM: LAPIC: Reset timer_advance

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Message-ID: <CANRm+CwHiZjh3w94Xdd=ZQXP6XWysz87OG+LFR2ekQn5A2P7Dw@mail.gmail.com>
Date:   Tue, 1 Sep 2020 11:25:01 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Reset timer_advance_ns if timer mode switch

On Mon, 31 Aug 2020 at 20:48, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Wanpeng Li <kernellwp@...il.com> writes:
>
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > 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