Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 10 Dec 2020 21:27:03 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Maxim Levitsky <mlevitsk@redhat.com>, 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>,
        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>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
Message-ID: <20201211002703.GA47016@fuller.cnet>
References: <20201203171118.372391-2-mlevitsk@redhat.com>
 <20201207232920.GD27492@fuller.cnet>
 <05aaabedd4aac7d3bce81d338988108885a19d29.camel@redhat.com>
 <87sg8g2sn4.fsf@nanos.tec.linutronix.de>
 <20201208181107.GA31442@fuller.cnet>
 <875z5c2db8.fsf@nanos.tec.linutronix.de>
 <20201209163434.GA22851@fuller.cnet>
 <87r1nyzogg.fsf@nanos.tec.linutronix.de>
 <20201210152618.GB23951@fuller.cnet>
 <87zh2lib8l.fsf@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87zh2lib8l.fsf@nanos.tec.linutronix.de>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

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.