Re: [virtio-dev] Re: [PATCH v1] virtio-mmio: Specify wait needed in driver during reset

[Jason Wang <jasowang@redhat.com>]

å 2021/8/16 äå1:35, Michael S. Tsirkin åé:
> On Mon, Aug 16, 2021 at 10:09:34AM +0800, Jason Wang wrote:
> > å 2021/8/11 äå6:05, Srivatsa Vaddagiri åé:
> > > * Jason Wang <jasowang@redhat.com> [2021-08-02 14:06:38]:
> > >
> > > > > Maybe if it sees status was not read it can just
> > > > > stay in reset state and not exit it?
> > > > Or I wonder we can use increase the version instead.
> > > What should be the guidance for backend/devices here? Go with version 3 only if
> > > it requires driver to poll on reset? Otherwise it may break existing setups -
> > > lets say Qemu is updated to report version 3 which will break older guest images
> > > that understood version 2 - this is despite the same Qemu being capable of
> > > serving those guests.
> >
> > My understanding is that qemu should stick to version 2 since it works fine.
> >
> > Thanks
> Yes breaking all guests does not sound like a good plan.
> Which is unfortunate generally.
>
> So thinking about all this, quite early in the setup process we
> have:
>
>          /* Figure out what features the device supports. */
>          device_features = dev->config->get_features(dev);
>
>
> isn't this sufficient? this will flush out
> all writes and device can defer responding to reads
> until reset is complete.


This works unless we mandate the deferring in the spec. And the racy 
part is here:

ÂÂÂÂÂÂÂ /* We always start by resetting the device, in case a previous
 * driver messed it up. This also tests that code path a 
little. */
ÂÂÂÂÂÂÂ dev->config->reset(dev);

ÂÂÂÂÂÂÂ /* Acknowledge that we've seen the device. */
ÂÂÂÂÂÂÂ virtio_add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);

If we don't read, the reset can be finished after add_status()?

Thanks


> I know it's not elegant since there are a bunch of writes like
> acknowledge and driver, but hey.
>
> How about we just add a non-normative section explaining that trick?
>
>
> > > - vatsa
> > >
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