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

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Message-Id: <FB43C4E2-D7C4-430B-9D6B-15FA59BB5286@amacapital.net>
Date:   Tue, 1 Dec 2020 08:19:29 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>, 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 Dec 1, 2020, at 6:01 AM, Thomas Gleixner <tglx@...utronix.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