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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

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.

> 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.

> 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?

> 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.

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.

Paolo