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

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Message-ID: <c7b9aef2-f5e0-d613-a17c-ef1a262b344c@redhat.com>
Date:   Tue, 27 Jun 2017 18:10:10 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2] KVM: LAPIC: Fix lapic timer injection delay



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.

Paolo