Re: [RFC PATCH v2 1/5] virtio: introduce virtio_force_modern()

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Nov 15 2021, Halil Pasic <pasic@linux.ibm.com> wrote:

> On Fri, 12 Nov 2021 16:37:20 +0100
> Cornelia Huck <cohuck@redhat.com> wrote:
>
>> On Fri, Nov 12 2021, Halil Pasic <pasic@linux.ibm.com> wrote:
>> 
>> > Legacy vs modern should be detected via transport specific means. We
>> > can't wait till feature negotiation is done. Let us introduce
>> > virtio_force_modern() as a means for the transport code to signal
>> > that the device should operate in modern mode (because a modern driver
>> > was detected).
>> >
>> > A new callback is added for the situations where the device needs
>> > to do more than just setting the VIRTIO_F_VERSION_1 feature bit. For
>> > example, when vhost is involved, we may need to propagate the features
>> > to the vhost device.
>> >
>> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
>> > ---
>> >
>> > I'm still struggling with how to deal with vhost-user and co. The
>> > problem is that I'm not very familiar with the life-cycle of, let us
>> > say, a vhost_user device.
>> >
>> > Looks to me like the vhost part might be just an implementation detail,
>> > and could even become a hot swappable thing.
>> >
>> > Another thing is, that vhost processes set_features differently. It
>> > might or might not be a good idea to change this.
>> >
>> > Does anybody know why don't we propagate the features on features_set,
>> > but under a set of different conditions, one of which is the vhost
>> > device is started?
>> > ---
>> >  hw/virtio/virtio.c         | 13 +++++++++++++
>> >  include/hw/virtio/virtio.h |  2 ++
>> >  2 files changed, 15 insertions(+)
>> >  
>> 
>> Did you see my feedback in
>> https://lore.kernel.org/qemu-devel/87tugzc26y.fsf@redhat.com/? I think
>> at least some of it still applies.
>> 
>
> Sure. My idea was to send out a v2 first which helps us think about the
> bigger picture, and then answer that mail. Now I realize I should have
> sent the response first, and then the v2 immediately afterwards.
>
>> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
>> > index 3a1f6c520c..26db1b31e6 100644
>> > --- a/hw/virtio/virtio.c
>> > +++ b/hw/virtio/virtio.c
>> > @@ -3281,6 +3281,19 @@ void virtio_init(VirtIODevice *vdev, const char *name,
>> >      vdev->use_guest_notifier_mask = true;
>> >  }
>> >  
>> > +void  virtio_force_modern(VirtIODevice *vdev)  
>> 
>> I'd still prefer to call this virtio_indicate_modern: we don't really
>> force anything; the driver has simply already decided that it will use
>> the modern interface and we provide an early indication in the features
>> so that code looking at them makes the right decisions.
>
> I tried to explain my dislike for virtio_indicate_modern in my response
> to that email. In somewhat different words: IMHO indication is about an
> external observer and has a symbolic dimension to it. Please see
> https://www.oxfordlearnersdictionaries.com/definition/english/indicate
> This function is about changing the behavior of the device. Its
> post-condition is: the device acts compliant to virtio 1.0 or higher.

My personal preference is "indicate", I don't like "force". I don't want
a semantics discussion; I'll leave the decision to the virtio
maintainers.

>
>> 
>> > +{
>> > +    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>> > +
>> > +    virtio_add_feature(&vdev->guest_features, VIRTIO_F_VERSION_1);
>> > +    /* Let the device do it's normal thing. */
>> > +    virtio_set_features(vdev, vdev->guest_features);  
>> 
>> I don't think this is substantially different from setting VERSION_1
>> only: At least the callers you introduce call this during reset,
>> i.e. when guest_features is 0 anyway. 
>
> I agree. Just wanted to be conservative, and preserve whatever is there.
>
>
>> We still have the whole processing
>> that is done after feature setting that may have effects different from
>> what the ultimate feature setting will give us.
>
> Yes, this is an intermediate state. As I pointed out, intermediate states
> are necessary.

Why? We just want VERSION_1 so that the checks work, why do we need to
fiddle with other settings? We only need to propagate it to e.g. vhost.

>
>> While I don't think
>> calling set_features twice is forbidden, that sequence is likely quite
>> untested, and I'm not sure we can exclude side effects.
>
> I can't disagree with that. But IMHO we can just say: such problems, if
> any, are bugs that need to be fixed.

Well, what about first fixing the endianness bugs, before we potentially
open up a can of worms?

>
> I think not doing the whole song-and-dance is conceptually more
> problematic because it is more likely to lead to inconsistent internal
> state. For example check out: vhost acked_features <-> guest_features.

What is wrong with verifying with one single feature?