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SubjectRe: [PATCH 0/2] RFC: Precise TSC migration
Date


> On Dec 1, 2020, at 6:01 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Mon, Nov 30 2020 at 16:16, Marcelo Tosatti wrote:
>> Not really. The synchronization logic tries to sync TSCs during
>> BIOS boot (and CPU hotplug), because the TSC values are loaded
>> sequentially, say:
>>
>> CPU realtime TSC val
>> vcpu0 0 usec 0
>> vcpu1 100 usec 0
>> vcpu2 200 usec 0
>
> That's nonsense, really.
>
>> And we'd like to see all vcpus to read the same value at all times.
>
> Providing guests with a synchronized and stable TSC on a host with a
> synchronized and stable TSC is trivial.
>
> Write the _same_ TSC offset to _all_ vcpu control structs and be done
> with it. It's not rocket science.
>
> The guest TSC read is:
>
> hostTSC + vcpu_offset
>
> So if the host TSC is synchronized then the guest TSCs are synchronized
> as well.
>
> If the host TSC is not synchronized, then don't even try.

This reminds me: if you’re adding a new kvm feature that tells the guest that the TSC works well, could you perhaps only have one structure for all vCPUs in the same guest?

>
> Thanks,
>
> tglx

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