linux-kernel - Re: [PATCH 2/2] x86, kvm: use kvmclock to compute TSC deadline value

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Message-ID: <CALzav=eR1eVv4kd7hyQ1J6TDNar4XniRY-V+cWLYb9qwBv6FAA@mail.gmail.com>
Date:   Fri, 9 Sep 2016 13:05:41 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Hornyack <peterhornyack@...gle.com>,
        X86 ML <x86@...nel.org>
Subject: Re: [PATCH 2/2] x86, kvm: use kvmclock to compute TSC deadline value

On Fri, Sep 9, 2016 at 9:38 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 09/09/2016 00:13, David Matlack wrote:
>> Hi Paolo,
>>
>> On Tue, Sep 6, 2016 at 3:29 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>> Bad things happen if a guest using the TSC deadline timer is migrated.
>>> The guest doesn't re-calibrate the TSC after migration, and the
>>> TSC frequency can and will change unless your processor supports TSC
>>> scaling (on Intel this is only Skylake) or your data center is perfectly
>>> homogeneous.
>>
>> Sorry, I forgot to follow up on our discussion in v1. One thing we
>> discussed there was using the APIC Timer to workaround a changing TSC
>> rate. You pointed out KVM's TSC deadline timer got a nice performance
>> boost recently, which makes it preferable. Does it makes sense to
>> apply the same optimization (using the VMX preemption timer) to the
>> APIC Timer, instead of creating a new dependency on kvmclock?
>
> Hi, yes it does.  If we go that way kvmclock.c should be patched to
> blacklist the TSC deadline timer.  However, I won't have time to work on
> it anytime soon, so _if I get reviews_ I'll take this patch first.

Got it, thanks for the context.