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

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.

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

         /* 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. */

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


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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