Stefan Hajnoczi <stefa...@gmail.com> writes:
> On Mon, 21 Nov 2022 at 09:49, Alex Bennée <alex.ben...@linaro.org> wrote: >> >> We have a bunch of variables associated with the device and the vhost >> backend which are used inconsistently throughout the code base. Lets >> start trying to bring some order by agreeing what each variable is >> for. Some cases to address (vho/vio renames to avoid ambiguous results >> while grepping): >> >> virtio->guest_features is mostly the live status of the features field >> and read and written as such by the guest. It does get manipulated by >> the various load state via virtio_set_features_nocheck(vdev, val) for >> migration. >> >> virtio->host_features is the result of vcd->get_features() most of the >> time and for vhost-user devices eventually ends up down at the vhost >> get features message: >> >> ./hw/virtio/virtio-bus.c:66: vdev->host_features = >> vdc->get_features(vdev, vdev->host_features, >> >> However virtio-net does a lot of direct modification of it: >> >> ./hw/net/virtio-net.c:3517: n->host_features |= (1ULL << >> VIRTIO_NET_F_MTU); >> ./hw/net/virtio-net.c:3529: n->host_features |= (1ULL << >> VIRTIO_NET_F_SPEED_DUPLEX); >> ./hw/net/virtio-net.c:3539: n->host_features |= (1ULL << >> VIRTIO_NET_F_SPEED_DUPLEX); >> ./hw/net/virtio-net.c:3548: n->host_features |= (1ULL << >> VIRTIO_NET_F_STANDBY); >> ./hw/virtio/virtio.c:3438: bool bad = (val & ~(vdev->host_features)) != >> 0; >> >> And we have this case which propagates the global QMP values for the >> device to the host features. This causes the resent regression of >> vhost-user-sock due to 69e1c14aa2 (virtio: core: vq reset feature >> negotation support) because the reset feature was rejected by the >> vhost-user backend causing it to freeze: >> >> ./hw/virtio/virtio.c:4667: status->host_features = >> qmp_decode_features(vdev->device_id, >> >> virtio->backend_features is only used by virtio-net to stash the >> vhost_net_get_features features for checking later: >> >> features = vhost_net_get_features(get_vhost_net(nc->peer), features); >> vdev->vio_backend_features = features; >> >> and: >> >> if (n->mtu_bypass_backend && >> !virtio_has_feature(vdev->vio_backend_features, >> VIRTIO_NET_F_MTU)) { >> features &= ~(1ULL << VIRTIO_NET_F_MTU); >> } >> >> vhost_dev->acked_features seems to mostly reflect >> virtio->guest_features (but where in the negotiation cycle?). Except >> for vhost_net where is becomes vhost_dev->backend_features >> >> ./backends/vhost-user.c:87: b->dev.vho_acked_features = >> b->vdev->guest_features; >> ./hw/block/vhost-user-blk.c:149: s->dev.vho_acked_features = >> vdev->guest_features; >> ./hw/net/vhost_net.c:132: net->dev.vho_acked_features = >> net->dev.vho_backend_features; >> ./hw/scsi/vhost-scsi-common.c:53: vsc->dev.vho_acked_features = >> vdev->guest_features; >> ./hw/virtio/vhost-user-fs.c:77: fs->vhost_dev.vho_acked_features = >> vdev->guest_features; >> ./hw/virtio/vhost-user-i2c.c:46: i2c->vhost_dev.vho_acked_features = >> vdev->guest_features; >> ./hw/virtio/vhost-user-rng.c:44: rng->vhost_dev.vho_acked_features = >> vdev->guest_features; >> ./hw/virtio/vhost-vsock-common.c:71: vvc->vhost_dev.vho_acked_features >> = vdev->guest_features; >> ./hw/virtio/vhost.c:1631: hdev->vho_acked_features |= bit_mask; >> >> vhost_dev->backend_features has become overloaded with two use cases: >> >> ./hw/block/vhost-user-blk.c:336: s->dev.vho_backend_features = 0; >> ./hw/net/vhost_net.c:180: net->dev.vho_backend_features = >> qemu_has_vnet_hdr(options->net_backend) >> ./hw/net/vhost_net.c:185: net->dev.vho_backend_features = 0; >> ./hw/scsi/vhost-scsi.c:220: vsc->dev.vho_backend_features = 0; >> ./hw/scsi/vhost-user-scsi.c:121: vsc->dev.vho_backend_features = 0; >> ./hw/virtio/vhost-user.c:2083: dev->vho_backend_features |= 1ULL << >> VHOST_USER_F_PROTOCOL_FEATURES; >> One use for saving the availability of a vhost-net feature and another >> for ensuring we add the protocol feature negotiation bit when querying >> a vhost backend. Maybe the places where this is queried should really >> be bools that can be queried in the appropriate places? >> >> Signed-off-by: Alex Bennée <alex.ben...@linaro.org> >> Cc: Stefano Garzarella <sgarz...@redhat.com> >> Cc: "Michael S. Tsirkin" <m...@redhat.com> >> Cc: Stefan Hajnoczi <stefa...@gmail.com> >> --- >> include/hw/virtio/vhost.h | 18 +++++++++++++++--- >> include/hw/virtio/virtio.h | 20 +++++++++++++++++++- >> 2 files changed, 34 insertions(+), 4 deletions(-) >> >> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h >> index 353252ac3e..502aa5677a 100644 >> --- a/include/hw/virtio/vhost.h >> +++ b/include/hw/virtio/vhost.h >> @@ -88,13 +88,25 @@ struct vhost_dev { >> int vq_index_end; >> /* if non-zero, minimum required value for max_queues */ >> int num_queues; >> + /** >> + * vhost feature handling requires matching the feature set >> + * offered by a backend which may be a subset of the total >> + * features eventually offered to the guest. >> + * >> + * @features: available features provided by the backend >> + * @acked_features: final set of negotiated features with the >> + * front-end driver >> + * @backend_features: additional feature bits applied during negotiation > > What does this mean? Well practically it is currently either applying VHOST_USER_F_PROTOCOL_FEATURES to the vhost_user_set_features() or storing VHOST_NET_F_VIRTIO_NET_HDR which I think eventually gets applied to: net->dev.acked_features = net->dev.backend_features; I suspect both could be dropped and handled as flags and applied at the destination. > >> + * >> + * Finally the @protocol_features is the final protocal feature > > s/protocal/protocol/ > > All the other fields are VIRTIO feature bits and this field holds the > VHOST_USER_SET_FEATURES feature bits? No these are the protocol features so a totally separate set of feature bits for the vhost user protocol. I don't think these apply to kernel vhost stuff? > >> + * set negotiated between QEMU and the backend (after >> + * VHOST_USER_F_PROTOCOL_FEATURES is negotiated) >> + */ >> uint64_t features; >> - /** @acked_features: final set of negotiated features */ >> uint64_t acked_features; >> - /** @backend_features: backend specific feature bits */ >> uint64_t backend_features; >> - /** @protocol_features: final negotiated protocol features */ >> uint64_t protocol_features; >> + >> uint64_t max_queues; >> uint64_t backend_cap; >> /* @started: is the vhost device started? */ >> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h >> index a973811cbf..9939a0a632 100644 >> --- a/include/hw/virtio/virtio.h >> +++ b/include/hw/virtio/virtio.h >> @@ -93,6 +93,12 @@ enum virtio_device_endian { >> VIRTIO_DEVICE_ENDIAN_BIG, >> }; >> >> +/** >> + * struct VirtIODevice - common VirtIO structure >> + * @name: name of the device >> + * @status: VirtIO Device Status field >> + * >> + */ >> struct VirtIODevice >> { >> DeviceState parent_obj; >> @@ -100,9 +106,21 @@ struct VirtIODevice >> uint8_t status; >> uint8_t isr; >> uint16_t queue_sel; >> - uint64_t guest_features; >> + /** >> + * These fields represent a set of VirtIO features at various >> + * levels of the stack. @host_features indicates the complete >> + * feature set the VirtIO device can offer to the driver. >> + * @guest_features indicates which features the VirtIO driver can >> + * support. > > The device never knows the features that the driver can support, so > this sentence is ambiguous/incorrect. The device only knows the > features that the driver writes during negotiation, which the spec > says is a subset of host_features. > > Maybe "indicates the features that driver wrote"? > > I noticed that this field is assigned even when the guest writes > invalid feature bits. Should we fix that? The negotiation sequence should be guest read, mask and write back so the value should be validated against host_features? > >> Finally @backend_features represents everything >> + * supported by the backend. This set might be split between stuff >> + * done by QEMU itself and stuff handled by an external backend >> + * (e.g. vhost). As a result some feature bits may be added or >> + * masked from the backend. > > I'm not 100% sure what this is referring to. Transport features that > are handled by QEMU and not the backend? Well there is the rub. While looking at the reset stuff it was postulated a device could support reset even if vhost part couldn't. If that is not true maybe we should drop this because host_features should have everything we need? > >> + */ >> uint64_t host_features; >> + uint64_t guest_features; >> uint64_t backend_features; >> + >> size_t config_len; >> void *config; >> uint16_t config_vector; >> -- >> 2.34.1 >> -- Alex Bennée