Re: What time is it kvm-clock?

From: Andy Lutomirski <luto@amacapital.net>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>,
	Owen Hofmann <osh@google.com>, KVM General <kvm@vger.kernel.org>,
	Peter Hornyack <peterhornyack@google.com>,
	Joao Martins <joao.m.martins@oracle.com>
Subject: Re: What time is it kvm-clock?
Date: Wed, 24 Feb 2016 11:52:10 -0800	[thread overview]
Message-ID: <CALCETrXWWskniE1X2hdgqxidYZMVhYstyjrKDJwVHYW0M0WaOw@mail.gmail.com> (raw)
In-Reply-To: <56CE082B.3020102@redhat.com>

On Wed, Feb 24, 2016 at 11:44 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> On 24/02/2016 20:38, Andy Lutomirski wrote:
>> If the master clock accurately exposed CLOCK_MONOTONIC_RAW or
>> CLOCK_MONOTONIC (I much prefer the latter), then it would be fine
>> across suspend/resume.
>
> Here we already have a conflict... Owen says he prefers the master clock
> to expose the (stable) TSC, you say you prefer CLOCK_MONOTONIC.
>
> I for one _thought_ CLOCK_MONOTONIC would have been my choice, but I'm
> not so sure about it and I'm also not sure it's possible to do it
> efficiently.

I think it should be configurable.

>  The mult/shift/offset tuple potentially can change every
> tick, and it would be bad to do such an update #vms times per tick (or
> worse, #vcpus times per tick).

If the interface were sane, it would be a single update per tick that
would update a data structure that all guests would share.  Arguably
we ought to expose all three useful clocks (MONOTONIC, MONOTONIC_RAW,
and REALTIME) in the same page.  The vclock code already does this for
MONOTONIC and REALTIME.)

Each vm would have a *different* page that had information like the
guest-boot-to-monotonic offset.

>
> Paolo

-- 
Andy Lutomirski
AMA Capital Management, LLC