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

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Message-ID: <754c46dd-3ead-2c27-1bcc-52db26418390@redhat.com>
Date:   Fri, 31 May 2019 11:01:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/2] KVM: LAPIC: Optimize timer latency consider world
 switch time

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.

Paolo