linux-kernel - Re: [PATCH 1/5] x86/kvm: On KVM re-enable (e.g. after suspend), update clocks

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Date:	Thu, 17 Mar 2016 20:58:20 +0100
From:	Radim Krcmar <rkrcmar@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Alexander Graf <agraf@...e.de>, kvm list <kvm@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>
Subject: Re: [PATCH 1/5] x86/kvm: On KVM re-enable (e.g. after suspend),
 update clocks

2016-03-17 11:22-0700, Andy Lutomirski:
> On Mar 17, 2016 8:10 AM, "Radim Krcmar" <rkrcmar@...hat.com> wrote:
>> 2016-03-16 16:07-0700, Andy Lutomirski:
>>> On Wed, Mar 16, 2016 at 3:59 PM, Radim Krcmar <rkrcmar@...hat.com> wrote:
>>>> 2016-03-16 15:15-0700, Andy Lutomirski:
>>>>> FWIW, if you ever intend to support ART ("always running timer")
>>>>> passthrough, this is going to be a giant clusterfsck.  Good luck.  I
>>>>> haven't gotten a straight answer as to what hardware actually supports
>>>>> that thing, so even testing isn't no easy.
>>>>
>>>> Hm, AR TSC would be best handled by doing nothing ... dropping the
>>>> faking logic just became tempting.
>>
>> ART is different from what I initially thought, it's the underlying
>> mechanism for invariant TSC and nothing more ...  we already forbid
>> migrations when the guest knows about invariant TSC, so we could do the
>> same and let ART be virtualized.  (Suspend has to be forbidden too.)
> 
> It's more than that -- it's a TSC-like clock that can be read by PCIe devices.

So ART is for time synchronization within the machine.  Makes sense now.

>>> As it stands, ART is screwed if you adjust the VMCS's tsc offset.  But
>>
>> Luckily, assigning real hardware can prevent migration or suspend, so we
>> won't need to adjust the offset during runtime.  TSC is a generally
>> unmigratable device that just happens to live on the CPU.
>>
>> (It would have been better to hide TSC capability from the guest and only
>>  use rdtsc for kvmclock if the guest wanted fancy features.)
>>
> 
> I think that, if KVM passes through an ART-supporting NIC, it might be
> rather messy to try to avoid passing through TSC as well.

I agree.  Migrating a guest with ART-supporting NIC is going to be hard
or impossible, so there is no big drawback in exposing TSC.

If KVM adds host TSC_ADJUST and VMCS TSC-offset to guest TSC_ADJUST,
then ART-supporting NIC should use timestamps compatible with VCPUs.

>                                                            But maybe a
> pvclock-like structure could expose the ART-kvmclock offset and scale.

I think that getting ART from kvmclock would turn out to be horrible.