linux-kernel - Re: [PATCH] kvm: kvmclock: let KVM_GET_CLOCK return whether the master clock is in use

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Message-ID: <20161109201716.GB25974@amt.cnet>
Date:   Wed, 9 Nov 2016 18:17:19 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] kvm: kvmclock: let KVM_GET_CLOCK return whether the
 master clock is in use

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?