Re: [Qemu-devel] -rtc base=, migration and time jumps

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Paolo Bonzini (pbonzini@redhat.com) wrote:
> So here's my understanding:  "-rtc base=" says what is the RTC value
> when the guest starts.  This value is only used by qemu_get_timedate,
> and most RTCs only use it on startup or reset.  However, there are
> exceptions (the PC RTC's host clock notifier, the ds1338's set time
> functionality, and all reads of m41t80/m48t59/twl92230) and this causes
> the bug.

Yes, I think so.

> On 19/07/19 14:36, Dr. David Alan Gilbert wrote:
> > d) The host clock jump detection (b) is broken - it correctly detects
> > backwards jumps; but it's detection of a forward jump is based
> > on two readings of the host clock being more than 60s apart - but
> > often ona q emu running a Linux guest the host clock isn't read at all;
> > so reading hwclock, waiting a minute and reading it again will trigger
> > the jump code.
> 
> Oops.  Back when the detection was added, there were two QEMU_CLOCK_HOST
> timers firing every second so the clock jump detection happened promptly.
> 
> These timers were then removed as a power-saving optimization, and that
> broke the jump detection.

Ah OK; I'm a bit cautious that perhaps we're still getting some benefit
from them; maybe in snapshots?

> > 1) Tell people to do what libvirt does and specify base= differently
> > on the dest.
> 
> This is racy; the user does not have a good way to know the exact base
> on the destination.

Right.

> > 2) Migrate the offset value such that the base= on the destination
> > is ignored
> 
> At least on some RTCs the offset is already being migrated indirectly.
> For example on x86 the (base_rtc, last_update) pair might be usable to
> reconstruct the offset?

Yes it probably is.

> > 3) Fix the host clock jump detection
> > 
> > (3) is probably independent - the easiest fix would seem to be just
> > to set a timer to read the host clock at say 20 second intervals
> > which is wasteful but would avoid the false trigger.
> > 
> > Is (2) worth it or do we just go with (1) - I'm tempted to just
> > specify the behaviour.
> > 
> > Mind you, we could kill the host clock jump detection code - only
> > the mc148618 registers on the notifier for it - so presumably
> > aarch/ppc/s390 etc dont see it.
> 
> I would just remove the host clock jump detection code.  IIUC that
> should fix your bug so you don't even need to do the above-mentioned
> reconstruction of the offset (let's call it 2b) in the PC RTC.

OK, I'll do that.

> That still leaves the problem that the base goes out of sync on
> migration on m41t80/m48t59/twl92230.  For that, I think that the
> simplest thing to do would be to fix those to store and migrate the
> offset themselves just like all other RTC implementations.

I'll put those on my TODO - I don't think they're actually used
by any versioned machine so keeping migration compat shouldn't
be an issue.

Dave

> Paolo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK