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

[Jason Wang <jasowang@redhat.com>]

On 2021/3/3 4:29 äå, Cornelia Huck wrote:
> On Wed, 3 Mar 2021 12:01:01 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
> > On 2021/3/2 8:08 äå, Cornelia Huck wrote:
> > > On Mon, 1 Mar 2021 11:51:08 +0800
> > > Jason Wang <jasowang@redhat.com> wrote:
> > >   
> > > > On 2021/3/1 5:25 äå, Michael S. Tsirkin wrote:
> > > > > On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:
> > > > > > On 2021/2/26 2:53 äå, 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[] = {
> >   ÂÂÂ {.flags = 1ULL << VIRTIO_NET_F_MAC,
> >   ÂÂÂÂ .end = endof(struct virtio_net_config, mac)},
> >   ÂÂÂ {.flags = 1ULL << VIRTIO_NET_F_STATUS,
> >   ÂÂÂÂ .end = endof(struct virtio_net_config, status)},
> >   ÂÂÂ {.flags = 1ULL << VIRTIO_NET_F_MQ,
> >   ÂÂÂÂ .end = endof(struct virtio_net_config, max_virtqueue_pairs)},
> >   ÂÂÂ {.flags = 1ULL << VIRTIO_NET_F_MTU,
> >   ÂÂÂÂ .end = endof(struct virtio_net_config, mtu)},
> >   ÂÂÂ {.flags = 1ULL << VIRTIO_NET_F_SPEED_DUPLEX,
> >   ÂÂÂÂ .end = endof(struct virtio_net_config, duplex)},
> >   ÂÂÂ {.flags = (1ULL << VIRTIO_NET_F_RSS) | (1ULL <<
> > VIRTIO_NET_F_HASH_REPORT),
> >   ÂÂÂÂ .end = endof(struct virtio_net_config, supported_hash_types)},
> >   ÂÂÂ {}
> > };*
> >
> > *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?


> This
> implies that a given field, if it exists, is always at the same offset
> from the beginning of the configuration space."
>
>
> > > (Do we need to specify what a device needs to do if the driver tries to
> > > access a non-existing field? We cannot really make assumptions about
> > > config space accesses; virtio-ccw can just copy a chunk of config space
> > > that contains non-existing fields...
> >
> > Right, it looks to me ccw doesn't depends on config len which is kind of
> > interesting. I think the answer depends on the implementation of both
> > transport and device:
> (virtio-ccw is a bit odd, because channel I/O does not have the concept
> of a config space and I needed to come up with something)


Ok.


> > Let's take virtio-net-pci as an example, it fills status unconditionally
> > in virtio_net_set_config() even if VIRTIO_NET_F_STATUS is not
> > negotiated. So with the above feature_sizes:
> >
> > 1) if one of the MQ, MTU, DUPLEX or RSS is implemented, driver can still
> > read status in this case
> > 2) if none of the above four is negotied, driver can only read a 0xFF
> > (virtio_config_readb())
> >
> >
> > > I guess the device could ignore
> > > writes and return zeroes on read?)
> >
> > So consider the above, it might be too late to fix/clarify that in the
> > spec on the expected behaviour of reading/writing non-exist fields.
> We could still advise behaviour via SHOULD, though I'm not sure it adds
> much at this point in time.
>
> It really feels a bit odd that a driver can still read and write fields
> for features that it did not negotiate, but I fear we're stuck with it.


Yes, since the device (at least virtio-pci) implment thing like this.

Thanks


> > Thanks
> >
> >
> > > I've opened https://github.com/oasis-tcs/virtio-spec/issues/98 for the
> > > spec issues.
> > >   
>
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