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

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Message-ID: <CALCETrVr2bM4yJTVpQULN+EYVQJuWGCvjX0SMFsCRy6BwqZc0w@mail.gmail.com>
Date:   Mon, 30 Nov 2020 08:54:19 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm list <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>,
        Thomas Gleixner <tglx@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        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 5:36 AM Maxim Levitsky <mlevitsk@...hat.com> wrote:
>
> Hi!
>
> This is the first version of the work to make TSC migration more accurate,
> as was defined by Paulo at:
> https://www.spinics.net/lists/kvm/msg225525.html
>
> I have a few thoughts about the kvm masterclock synchronization,
> which relate to the Paulo's proposal that I implemented.
>
> The idea of masterclock is that when the host TSC is synchronized
> (or as kernel call it, stable), and the guest TSC is synchronized as well,
> then we can base the kvmclock, on the same pair of
> (host time in nsec, host tsc value), for all vCPUs.
>
> This makes the random error in calculation of this value invariant
> across vCPUS, and allows the guest to do kvmclock calculation in userspace
> (vDSO) since kvmclock parameters are vCPU invariant.
>
> To ensure that the guest tsc is synchronized we currently track host/guest tsc
> writes, and enable the master clock only when roughly the same guest's TSC value
> was written across all vCPUs.
>
> Recently this was disabled by Paulo and I agree with this, because I think
> that we indeed should only make the guest TSC synchronized by default
> (including new hotplugged vCPUs) and not do any tsc synchronization beyond that.
> (Trying to guess when the guest syncs the TSC can cause more harm that good).
>
> Besides, Linux guests don't sync the TSC via IA32_TSC write,
> but rather use IA32_TSC_ADJUST which currently doesn't participate
> in the tsc sync heruistics.
> And as far as I know, Linux guest is the primary (only?) user of the kvmclock.
>
> I *do think* however that we should redefine KVM_CLOCK_TSC_STABLE
> in the documentation to state that it only guarantees invariance if the guest
> doesn't mess with its own TSC.
>
> Also I think we should consider enabling the X86_FEATURE_TSC_RELIABLE
> in the guest kernel, when kvm is detected to avoid the guest even from trying
> to sync TSC on newly hotplugged vCPUs.
>
> (The guest doesn't end up touching TSC_ADJUST usually, but it still might
> in some cases due to scheduling of guest vCPUs)
>
> (X86_FEATURE_TSC_RELIABLE short circuits tsc synchronization on CPU hotplug,
> and TSC clocksource watchdog, and the later we might want to keep).

If you're going to change the guest behavior to be more trusting of
the host, I think
the host should probably signal this to the guest using a new bit.