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

From: David Matlack
Date: Fri Sep 09 2016 - 16:06:23 EST

On Fri, Sep 9, 2016 at 9:38 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> On 09/09/2016 00:13, David Matlack wrote:
>> Hi Paolo,
>> On Tue, Sep 6, 2016 at 3:29 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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.