Re: [Qemu-devel] pl031 time across vm save/reload

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

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On Fri, 5 Jul 2019 at 11:13, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > On 05/07/19 11:58, Peter Maydell wrote:
> > > On Fri, 5 Jul 2019 at 10:48, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > >> You're right, the compatibility causes wrong behavior for the default
> > >> -rtc settings (the RC pauses across migration).  The right thing to do
> > >> would be to store the base rather than the offset: that is, you store
> > >> the time at which LR was written.  Then the offset is s->lr - s->base
> > >> and it's independent of the machine on which the rtc_clock is being read.
> > >
> > > Right. How do we handle this for back-compat purposes? I guess
> > > we need to have a new migration subsection, so if it's present
> > > it has the 'base' value and we ignore the 'offset' in the
> > > main migration data, and if it's not present we assume an
> > > old->new migration and use the existing offset code. New->old
> > > migration would not be possible as the new subsection is
> > > always-present.
> >
> > Yes, something like that but I would just bump the version.  Version 1
> > has the old meaning for the first field, version 2 has the new meaning.
> 
> Yeah, we could do that. I thought we preferred to avoid using
> version-numbers for migration though these days ? (cc'ing DG
> in case he has an opinion.)

Right.
Add a subsection, make the subsection only be sent if you're on a new
machine type.

(I'm currently getting my head around our x86 RTC code because of a bug
I've been handed involving RTCs and migration; the expectations and the
behaviours are not obvious at all).

Dave

> > And also, since our brains are fresh on pl031... currently s->lr is
> > always 0; besides the bug that writing RTC_LR should update it, the
> > datasheet says the counter counts up from 1 so perhaps at startup s->lr
> > should be set to a nonzero value?   That would be
> > qemu_ref_timedate(QEMU_CLOCK_VIRTUAL) - 1.
> 
> The 'summary of RTC registers' section in the datasheet says
> that RTCLR's reset value is zero...
> 
> thanks
> -- PMM
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK