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

[Paolo Bonzini]

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.

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.

Paolo