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

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Date:   Wed, 9 Nov 2016 17:03:55 -0500 (EST)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...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



----- Original Message -----
> 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>
> Sent: Wednesday, November 9, 2016 9:17:19 PM
> 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).

__get_kernel_ns now computes the guest's kvmclock directly when using the
master clock.  Instead of peeking at guest memory, it does it from the
hypervisor's copy of the parameters.

Paolo