Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero

[Jason Wang]

On 2021/3/4 9:50 下午, Cornelia Huck wrote:
> On Thu, 4 Mar 2021 16:24:16 +0800
> Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> > On 2021/3/3 4:29 下午, Cornelia Huck wrote:
> > > On Wed, 3 Mar 2021 12:01:01 +0800
> > > Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > >   
> > > > On 2021/3/2 8:08 下午, Cornelia Huck wrote:
> > > > > On Mon, 1 Mar 2021 11:51:08 +0800
> > > > > Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > > > >      
> > > > > > On 2021/3/1 5:25 上午, Michael S. Tsirkin wrote:
> > > > > > > On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:
> > > > > > > > On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:
> > > > > > > > > Confused. What is wrong with the above? It never reads the
> > > > > > > > > field unless the feature has been offered by device.
> > > > > > > > So the spec said:
> > > > > > > >
> > > > > > > > "
> > > > > > > >
> > > > > > > > The following driver-read-only field, max_virtqueue_pairs only exists if
> > > > > > > > VIRTIO_NET_F_MQ is set.
> > > > > > > >
> > > > > > > > "
> > > > > > > >
> > > > > > > > If I read this correctly, there will be no max_virtqueue_pairs field if the
> > > > > > > > VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
> > > > > > > > what spec said.
> > > > > > > >
> > > > > > > > Thanks
> > > > > > > I think that's a misunderstanding. This text was never intended to
> > > > > > > imply that field offsets change beased on feature bits.
> > > > > > > We had this pain with legacy and we never wanted to go back there.
> > > > > > >
> > > > > > > This merely implies that without VIRTIO_NET_F_MQ the field
> > > > > > > should not be accessed. Exists in the sense "is accessible to driver".
> > > > > > >
> > > > > > > Let's just clarify that in the spec, job done.
> > > > > > Ok, agree. That will make things more eaiser.
> > > > > Yes, that makes much more sense.
> > > > >
> > > > > What about adding the following to the "Basic Facilities of a Virtio
> > > > > Device/Device Configuration Space" section of the spec:
> > > > >
> > > > > "If an optional configuration field does not exist, the corresponding
> > > > > space is still present, but reserved."
> > > > This became interesting after re-reading some of the qemu codes.
> > > >
> > > > E.g in virtio-net.c we had:
> > > >
> > > > *static VirtIOFeature feature_sizes[] = {
> > > >        {.flags = 1ULL << VIRTIO_NET_F_MAC,
> > > >         .end = endof(struct virtio_net_config, mac)},
> > > >        {.flags = 1ULL << VIRTIO_NET_F_STATUS,
> > > >         .end = endof(struct virtio_net_config, status)},
> > > >        {.flags = 1ULL << VIRTIO_NET_F_MQ,
> > > >         .end = endof(struct virtio_net_config, max_virtqueue_pairs)},
> > > >        {.flags = 1ULL << VIRTIO_NET_F_MTU,
> > > >         .end = endof(struct virtio_net_config, mtu)},
> > > >        {.flags = 1ULL << VIRTIO_NET_F_SPEED_DUPLEX,
> > > >         .end = endof(struct virtio_net_config, duplex)},
> > > >        {.flags = (1ULL << VIRTIO_NET_F_RSS) | (1ULL <<
> > > > VIRTIO_NET_F_HASH_REPORT),
> > > >         .end = endof(struct virtio_net_config, supported_hash_types)},
> > > >        {}
> > > > };*
> > > >
> > > > *It has a implict dependency chain. E.g MTU doesn't presnet if
> > > > DUPLEX/RSS is not offered ...
> > > > *
> > > But I think it covers everything up to the relevant field, no? So MTU
> > > is included if we have the feature bit, even if we don't have
> > > DUPLEX/RSS.
> > >
> > > Given that a config space may be shorter (but must not collapse
> > > non-existing fields), maybe a better wording would be:
> > >
> > > "If an optional configuration field does not exist, the corresponding
> > > space will still be present if it is not at the end of the
> > > configuration space (i.e., further configuration fields exist.)
> >
> > This should work but I think we need to define the end of configuration
> > space first?
> What about sidestepping this:
>
> "...the corresponding space will still be present, unless no further
> configuration fields exist."
>
> ?


It might work. (I wonder maybe we can give some example in the spec).

Thanks


> > > This
> > > implies that a given field, if it exists, is always at the same offset
> > > from the beginning of the configuration space."