linux-kernel - Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC

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Message-ID: <20201211002703.GA47016@fuller.cnet>
Date:   Thu, 10 Dec 2020 21:27:03 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

On Thu, Dec 10, 2020 at 10:48:10PM +0100, Thomas Gleixner wrote:
> On Thu, Dec 10 2020 at 12:26, Marcelo Tosatti wrote:
> > On Wed, Dec 09, 2020 at 09:58:23PM +0100, Thomas Gleixner wrote:
> >> Marcelo,
> >> 
> >> On Wed, Dec 09 2020 at 13:34, Marcelo Tosatti wrote:
> >> > On Tue, Dec 08, 2020 at 10:33:15PM +0100, Thomas Gleixner wrote:
> >> >> On Tue, Dec 08 2020 at 15:11, Marcelo Tosatti wrote:
> >> >> > max_cycles overflow. Sent a message to Maxim describing it.
> >> >> 
> >> >> Truly helpful. Why the hell did you not talk to me when you ran into
> >> >> that the first time?
> >> >
> >> > Because 
> >> >
> >> > 1) Users wanted CLOCK_BOOTTIME to stop counting while the VM 
> >> > is paused (so we wanted to stop guest clock when VM is paused anyway).
> >> 
> >> How is that supposed to work w/o the guest kernels help if you have to
> >> keep clock realtime up to date? 
> >
> > Upon VM resume, we notify NTP daemon in the guest to sync realtime
> > clock.
> 
> Brilliant. What happens if there is no NTP daemon? What happens if the
> NTP daemon is not part of the virt orchestration magic and cannot be
> notified, then it will notice the time jump after the next update
> interval.
> 
> What about correctness?
> 
> ALL CLOCK_* stop and resume when the VM is resumed at the point where
> they stopped.
> 
> So up to the point where NTP catches up and corrects clock realtime and
> TAI other processes can observe that time jumped in the outside world,
> e.g. via a network packet or whatever, but there is no reason why time
> should have jumped outside vs. the local one.
> 
> You really all live in a seperate universe creating your own rules how
> things which other people work hard on to get it correct can be screwed
> over.

	1. T = read timestamp.
	2. migrate (VM stops for a certain period).
	3. use timestamp T.

> Of course this all is nowhere documented in detail. At least a quick
> search with about 10 different keyword combinations revealed absolutely
> nothing.
> 
> This features first, correctness later frenzy is insane and it better
> stops now before you pile even more crap on the existing steaming pile
> of insanities.

Sure.