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

[Peter Maydell]

On Fri, 5 Jul 2019 at 11:13, Paolo Bonzini <address@hidden> wrote:
>
> On 05/07/19 11:58, Peter Maydell wrote:
> > On Fri, 5 Jul 2019 at 10:48, Paolo Bonzini <address@hidden> 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.)

> 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