Date: Wed, 9 Nov 2016 19:14:07 -0200
From: Marcelo Tosatti <mtosatti@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Subject: Re: [PATCH] kvm: kvmclock: let KVM_GET_CLOCK return whether the
 master clock is in use
Message-ID: <20161109211404.GA2218@amt.cnet>
References: <1478710095-15022-1-git-send-email-pbonzini@redhat.com>
 <20161109201248.GA25974@amt.cnet>
 <20161109201716.GB25974@amt.cnet>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161109201716.GB25974@amt.cnet>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1236
Lines: 35

On Wed, Nov 09, 2016 at 06:17:16PM -0200, Marcelo Tosatti wrote:
> On Wed, Nov 09, 2016 at 06:12:50PM -0200, Marcelo Tosatti wrote:
> > On Wed, Nov 09, 2016 at 05:48:15PM +0100, Paolo Bonzini wrote:
> > > Userspace can read the exact value of kvmclock by reading the TSC
> > > and fetching the timekeeping parameters out of guest memory.  This
> > > however is brittle and not necessary anymore with KVM 4.11.  
> > 
> > Hi Paolo,
> > 
> > Can you point to commit or explanation why that is not the case anymore?
> > Thanks
> 
> I don't see how thats possible given the:
> 
>  * TSC 
>  * TSC (timer interrupt + TSC deltas) (AKA host TSC clocksource and
>  * CLOCK_MONOTONIC).
>  
> Clocks currently drift from each other (therefore are not monotonic).
> 
> Have you confirmed CLOCK_MONOTONIC_RAW and TSC are monotonic?

I think the benefits of CLOCK_MONOTONIC_RAW (which supposedly 
is monotonic with TSC for large lenghts of time) outweight the 
downsides (just have to check that).

The downsides are that on migration from

    source: TSC with freq Y.
    dest: TSC with freq X.

Suddenly kvmclock starts drifting at a different rate, 
unsure how well Windows time service and 
older NTPd respond to this (might be a regression).