linux-kernel - Re: [PATCH 1/2] KVM: LAPIC: Optimize timer latency consider world switch time

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Message-ID: <CANRm+Czg+5qe3eHrOo-=EbOJM5929Xge+rSwyQoksdkk6PcGzA@mail.gmail.com>
Date:   Wed, 12 Jun 2019 17:38:45 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/2] KVM: LAPIC: Optimize timer latency consider world
 switch time

On Fri, 31 May 2019 at 17:01, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 30/05/19 21:36, Sean Christopherson wrote:
> >> +u32 __read_mostly vmentry_lapic_timer_advance_ns = 0;
> >> +module_param(vmentry_lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
> > Hmm, an interesting idea would be to have some way to "lock" this param,
> > e.g. setting bit 0 locks the param.  That would allow KVM to calculate the
> > cycles value to avoid the function call and the MUL+DIV.  If I'm not
> > mistaken, vcpu->arch.virtual_tsc_khz is set only in kvm_set_tsc_khz().
>
> I would just make it read-only.  But I'm afraid we're entering somewhat
> dangerous territory.  There is a risk that the guest ends up entering
> the interrupt handler before the TSC deadline has actually expired, and
> there would be no way to know what would happen; even guest hangs are
> possible.

Agreed, do it in v3.

Regards,
Wanpeng Li