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