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Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Thomas Gleixner <tglx@linutronix.de>, kvm@vger.kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Jim Mattson <jmattson@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        open list <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@alien8.de>,
        Shuah Khan <shuah@kernel.org>,
        Andrew Jones <drjones@redhat.com>,
        Oliver Upton <oupton@google.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Date:   Tue, 08 Dec 2020 13:24:11 +0200
In-Reply-To: <885C1725-B479-47F6-B08D-A7181637A80A@amacapital.net>
References: <636fecc20b0143128b484f159ff795ff65d05b82.camel@redhat.com>
         <885C1725-B479-47F6-B08D-A7181637A80A@amacapital.net>
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On Mon, 2020-12-07 at 10:04 -0800, Andy Lutomirski wrote:
> > On Dec 7, 2020, at 9:00 AM, Maxim Levitsky <mlevitsk@redhat.com> wrote:
> > 
> > ﻿On Mon, 2020-12-07 at 08:53 -0800, Andy Lutomirski wrote:
> > > > > On Dec 7, 2020, at 8:38 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > > > 
> > > > ﻿On Mon, Dec 07 2020 at 14:16, Maxim Levitsky wrote:
> > > > > > On Sun, 2020-12-06 at 17:19 +0100, Thomas Gleixner wrote:
> > > > > > From a timekeeping POV and the guests expectation of TSC this is
> > > > > > fundamentally wrong:
> > > > > > 
> > > > > >     tscguest = scaled(hosttsc) + offset
> > > > > > 
> > > > > > The TSC has to be viewed systemwide and not per CPU. It's systemwide
> > > > > > used for timekeeping and for that to work it has to be synchronized. 
> > > > > > 
> > > > > > Why would this be different on virt? Just because it's virt or what? 
> > > > > > 
> > > > > > Migration is a guest wide thing and you're not migrating single vCPUs.
> > > > > > 
> > > > > > This hackery just papers over he underlying design fail that KVM looks
> > > > > > at the TSC per vCPU which is the root cause and that needs to be fixed.
> > > > > 
> > > > > I don't disagree with you.
> > > > > As far as I know the main reasons that kvm tracks TSC per guest are
> > > > > 
> > > > > 1. cases when host tsc is not stable 
> > > > > (hopefully rare now, and I don't mind making
> > > > > the new API just refuse to work when this is detected, and revert to old way
> > > > > of doing things).
> > > > 
> > > > That's a trainwreck to begin with and I really would just not support it
> > > > for anything new which aims to be more precise and correct.  TSC has
> > > > become pretty reliable over the years.
> > > > 
> > > > > 2. (theoretical) ability of the guest to introduce per core tsc offfset
> > > > > by either using TSC_ADJUST (for which I got recently an idea to stop
> > > > > advertising this feature to the guest), or writing TSC directly which
> > > > > is allowed by Intel's PRM:
> > > > 
> > > > For anything halfways modern the write to TSC is reflected in TSC_ADJUST
> > > > which means you get the precise offset.
> > > > 
> > > > The general principle still applies from a system POV.
> > > > 
> > > >    TSC base (systemwide view) - The sane case
> > > > 
> > > >    TSC CPU  = TSC base + TSC_ADJUST
> > > > 
> > > > The guest TSC base is a per guest constant offset to the host TSC.
> > > > 
> > > >    TSC guest base = TSC host base + guest base offset
> > > > 
> > > > If the guest want's this different per vCPU by writing to the MSR or to
> > > > TSC_ADJUST then you still can have a per vCPU offset in TSC_ADJUST which
> > > > is the offset to the TSC base of the guest.
> > > 
> > > How about, if the guest wants to write TSC_ADJUST, it can turn off all paravirt features and keep both pieces?
> > > 
> > 
> > This is one of the things I had in mind recently.
> > 
> > Even better, we can stop advertising TSC_ADJUST in CPUID to the guest 
> > and forbid it from writing it at all.
> 
> Seems reasonable to me.
> 
> It also seems okay for some MSRs to stop working after the guest enabled new PV timekeeping.
This is a very good idea!

> 
> I do have a feature request, though: IMO it would be quite nifty if the new kvmclock structure could also expose NTP corrections. In other words, if you could expose enough info to calculate CLOCK_MONOTONIC_RAW, CLOCK_MONOTONIC, and CLOCK_REALTIME, then we could have paravirt NTP.
> 
> Bonus points if whatever you do for CLOCK_REALTIME also exposes leap seconds in a race free way :). But I suppose that just exposing TAI and letting the guest deal with the TAI - UTC offset itself would get the job done just fine.

This is a good idea too.
As I understand it, this gives a justification to a new kvmclock purpose,
which wouldn't be focused anymore on correcting the tsc shortcomings
(unstable/unscalable tsc), but more on things like that.
I like that idea.

Best regards,
	Maxim Levitsky