From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-dev-return-8098-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 2FB939864CE for ; Thu, 4 Mar 2021 13:50:15 +0000 (UTC) Date: Thu, 4 Mar 2021 14:50:00 +0100 From: Cornelia Huck Message-ID: <20210304145000.149706ae.cohuck@redhat.com> In-Reply-To: <1b5b3f9b-41d7-795c-c677-c45f1d5a774e@redhat.com> References: <20210223041740-mutt-send-email-mst@kernel.org> <788a0880-0a68-20b7-5bdf-f8150b08276a@redhat.com> <20210223110430.2f098bc0.cohuck@redhat.com> <20210223115833.732d809c.cohuck@redhat.com> <8355f9b3-4cda-cd2e-98df-fed020193008@redhat.com> <20210224121234.0127ae4b.cohuck@redhat.com> <20210225135229-mutt-send-email-mst@kernel.org> <0f8eb381-cc98-9e05-0e35-ccdb1cbd6119@redhat.com> <20210228162306-mutt-send-email-mst@kernel.org> <20210302130812.6227f176.cohuck@redhat.com> <5f6972fe-7246-b622-958d-9cab8dd98e21@redhat.com> <20210303092905.677eb66c.cohuck@redhat.com> <1b5b3f9b-41d7-795c-c677-c45f1d5a774e@redhat.com> MIME-Version: 1.0 Subject: Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable To: Jason Wang Cc: "Michael S. Tsirkin" , Si-Wei Liu , elic@nvidia.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, virtio-dev@lists.oasis-open.org List-ID: On Thu, 4 Mar 2021 16:24:16 +0800 Jason Wang wrote: > On 2021/3/3 4:29 =E4=B8=8B=E5=8D=88, Cornelia Huck wrote: > > On Wed, 3 Mar 2021 12:01:01 +0800 > > Jason Wang wrote: > > =20 > >> On 2021/3/2 8:08 =E4=B8=8B=E5=8D=88, Cornelia Huck wrote: =20 > >>> On Mon, 1 Mar 2021 11:51:08 +0800 > >>> Jason Wang wrote: > >>> =20 > >>>> On 2021/3/1 5:25 =E4=B8=8A=E5=8D=88, Michael S. Tsirkin wrote: =20 > >>>>> On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote: =20 > >>>>>> On 2021/2/26 2:53 =E4=B8=8A=E5=8D=88, Michael S. Tsirkin wrote: = =20 > >>>>>>> Confused. What is wrong with the above? It never reads the > >>>>>>> field unless the feature has been offered by device. =20 > >>>>>> So the spec said: > >>>>>> > >>>>>> " > >>>>>> > >>>>>> The following driver-read-only field, max_virtqueue_pairs only exi= sts if > >>>>>> VIRTIO_NET_F_MQ is set. > >>>>>> > >>>>>> " > >>>>>> > >>>>>> If I read this correctly, there will be no max_virtqueue_pairs fie= ld if the > >>>>>> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() vi= olates > >>>>>> what spec said. > >>>>>> > >>>>>> Thanks =20 > >>>>> 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 drive= r". > >>>>> > >>>>> Let's just clarify that in the spec, job done. =20 > >>>> Ok, agree. That will make things more eaiser. =20 > >>> 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." =20 > >> > >> This became interesting after re-reading some of the qemu codes. > >> > >> E.g in virtio-net.c we had: > >> > >> *static VirtIOFeature feature_sizes[] =3D { > >> =C2=A0=C2=A0=C2=A0 {.flags =3D 1ULL << VIRTIO_NET_F_MAC, > >> =C2=A0=C2=A0=C2=A0=C2=A0 .end =3D endof(struct virtio_net_config, ma= c)}, > >> =C2=A0=C2=A0=C2=A0 {.flags =3D 1ULL << VIRTIO_NET_F_STATUS, > >> =C2=A0=C2=A0=C2=A0=C2=A0 .end =3D endof(struct virtio_net_config, st= atus)}, > >> =C2=A0=C2=A0=C2=A0 {.flags =3D 1ULL << VIRTIO_NET_F_MQ, > >> =C2=A0=C2=A0=C2=A0=C2=A0 .end =3D endof(struct virtio_net_config, ma= x_virtqueue_pairs)}, > >> =C2=A0=C2=A0=C2=A0 {.flags =3D 1ULL << VIRTIO_NET_F_MTU, > >> =C2=A0=C2=A0=C2=A0=C2=A0 .end =3D endof(struct virtio_net_config, mt= u)}, > >> =C2=A0=C2=A0=C2=A0 {.flags =3D 1ULL << VIRTIO_NET_F_SPEED_DUPLEX, > >> =C2=A0=C2=A0=C2=A0=C2=A0 .end =3D endof(struct virtio_net_config, du= plex)}, > >> =C2=A0=C2=A0=C2=A0 {.flags =3D (1ULL << VIRTIO_NET_F_RSS) | (1ULL << > >> VIRTIO_NET_F_HASH_REPORT), > >> =C2=A0=C2=A0=C2=A0=C2=A0 .end =3D endof(struct virtio_net_config, su= pported_hash_types)}, > >> =C2=A0=C2=A0=C2=A0 {} > >> };* > >> > >> *It has a implict dependency chain. E.g MTU doesn't presnet if > >> DUPLEX/RSS is not offered ... > >> * =20 > > 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.) =20 >=20 >=20 > This should work but I think we need to define the end of configuration= =20 > space first? What about sidestepping this: "...the corresponding space will still be present, unless no further configuration fields exist." ? >=20 > > This > > implies that a given field, if it exists, is always at the same offset > > from the beginning of the configuration space." --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org