Messages in this thread | | | Subject | Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero | From | Jason Wang <> | Date | Thu, 4 Mar 2021 16:24:16 +0800 |
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On 2021/3/3 4:29 下午, Cornelia Huck wrote: > On Wed, 3 Mar 2021 12:01:01 +0800 > Jason Wang <jasowang@redhat.com> wrote: > >> On 2021/3/2 8:08 下午, Cornelia Huck wrote: >>> On Mon, 1 Mar 2021 11:51:08 +0800 >>> Jason Wang <jasowang@redhat.com> 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?
> 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. >>> > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org >
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