linux-kernel - Re: [PATCH 0/2] RFC: Precise TSC migration

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Message-ID: <874kl5hbgp.fsf@nanos.tec.linutronix.de>
Date:   Tue, 01 Dec 2020 15:01:26 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 0/2] RFC: Precise TSC migration

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.

Thanks,

        tglx