Messages in this thread | | | Date | Thu, 17 Mar 2016 20:58:20 +0100 | From | Radim Krcmar <> | Subject | Re: [PATCH 1/5] x86/kvm: On KVM re-enable (e.g. after suspend), update clocks |
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2016-03-17 11:22-0700, Andy Lutomirski: > On Mar 17, 2016 8:10 AM, "Radim Krcmar" <rkrcmar@redhat.com> wrote: >> 2016-03-16 16:07-0700, Andy Lutomirski: >>> On Wed, Mar 16, 2016 at 3:59 PM, Radim Krcmar <rkrcmar@redhat.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.
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