Re: [Qemu-arm] [PATCH v3 07/33] automatically add vmstate for reset support in devices

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

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On Thu, 8 Aug 2019 at 16:42, Dr. David Alan Gilbert <dgilbert@redhat.com> wrote:
> >
> > * Peter Maydell (peter.maydell@linaro.org) wrote:
> > > On Mon, 29 Jul 2019 at 15:59, Damien Hedde <damien.hedde@greensocs.com> wrote:
> > > >
> > > > This add the reset related sections for every QOM
> > > > device.
> > >
> > > A bit more detail in the commit message would help, I think --
> > > this is adding extra machinery which has to copy and modify
> > > the VMStateDescription passed in by the device in order to
> > > add the subsection that handles reset.
> > >
> > > I've added Dave Gilbert to the already long cc list since this
> > > is migration related.
> >
> > I don't like dynamically modifying all the vmsds.
> 
> Yeah, I'm not a huge fan of it either, but it does mean that
> the state gets migrated and we don't have a pile of really
> easy to miss bugs where we forgot to say "this device needs to
> migrate the reset state" and it means we don't have every
> device we ever write in the future needing to say "this device
> needs to migrate reset state"...
> 
> > Aren't you going to have to understand each devices reset behaviour
> > and make sure it does something sane? e.g. it might have a postload
> > that registers a timer or something that you wouldn't want to do if it's
> > in reset.
> >
> > The easiest way is to write a macro that you can easily add to devices
> > you've checked subsection list (like the way we have a
> > VMSTATE_USB_DEVICE).
> 
> One problem is that as soon as somebody writes a USB controller
> or PCI controller model that can be held in reset, every
> device that could sat behind it automatically now could find
> that it's migrated while it's in reset.

I'm not convinced though that they'll all be fixed by adding this
subsection; I think some of those devices you're going to have to
look at and see what they do.

Dave

> thanks
> -- PMM
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK