Re: Timekeeping on ARM guests/hosts

[Marc Zyngier <marc.zyngier@arm.com>]

Hi Miriam,

On Wed, 10 Oct 2018 19:38:47 +0100,
Miriam Zimmerman <mutexlox@google.com> wrote:
> 
> (oops, sorry for lack of plaintext in the first email. must've
> forgotten to click the button in my email client)
> 
> Until that happens, what's the best workaround? Just running an ntp
> daemon in guest?

Christoffer reminded me yesterday that stolen time accounting only
affects scheduling, and is not evaluated for

An NTP daemon may not be the best course of action, as the guest is
going to see a massive jump anyway, which most NTP implementations are
not design to handle (they rightly assume that something else is
wrong). It would also mean that you'd have to run a NTP server
somewhere on the host, as you cannot always assume full connectivity.

A popular way to solve this seems to be using the QEMU guest agent,
but I must admit I never really investigated that side of the problem.

I'm quite curious of how this is done on x86 though. KVM_GUEST mostly
seems to give the guest a PV clocksource, which is not going to help in
terms of wall clock. Do you have any idea?

Thanks,

	M.

> 
> On Wed, Oct 10, 2018 at 3:01 AM Marc Zyngier <marc.zyngier@arm.com> wrote:
> >
> > Hi Myriam,
> >
> > On 10/10/18 00:39, Miriam Zimmerman wrote:
> > > Hi,
> > >
> > > I'm working with an ARM device hosting an ARM guest. When the host is
> > > suspended, guest time stops advancing and it doesn't get adjusted on resume.
> >
> > I know the feeling, my arm64 laptop gives me that kind of grief all the
> > time... :-/
> >
> > > For an x86 machine, the CONFIG_KVM_GUEST flag would enable paravirt for
> > > time and fix this problem, but CONFIG_KVM_GUEST isn't available on ARM.
> > >
> > > Is there a configuration option to enable paravirtualized timekeeping on
> > > ARM? If not, how can I configure ARM guests to handle timekeeping properly?
> >
> > PV time (or rather stolen time) is a work in progress at the moment, and
> > Christoffer has his hands in that particular pie.
> >
> > Thanks,
> >
> >         M.
> > --
> > Jazz is not dead. It just smells funny...

-- 
Jazz is not dead, it just smell funny.