Acked-by: Raphael Norwitz <[email protected]>
On Sat, Oct 11, 2025 at 7:24 PM Vladimir Sementsov-Ogievskiy
<[email protected]> wrote:
>
> This field is mostly unused and sometimes confusing (we even have
> a TODO-like comment to drop it). Let's finally do.
>
> The field is used to held VHOST_USER_F_PROTOCOL_FEATURES for vhost-user
> and/or VHOST_NET_F_VIRTIO_NET_HDR for vhost-net (which may be
> vhost-user-net). But we can simply recalculate these two flags in place
> from hdev->features, and from net-client for VHOST_NET_F_VIRTIO_NET_HDR.
>
> Note: removing field from x-query-virtio-status result is incompatible
> change. We can do it because the command is unstable.
>
> Cc: [email protected]
> Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
> ---
> hw/block/vhost-user-blk.c | 1 -
> hw/net/vhost_net.c | 26 ++++++++++++++------------
> hw/scsi/vhost-scsi.c | 1 -
> hw/scsi/vhost-user-scsi.c | 1 -
> hw/virtio/vdpa-dev.c | 1 -
> hw/virtio/vhost-user.c | 17 ++++++++---------
> hw/virtio/virtio-hmp-cmds.c | 2 --
> hw/virtio/virtio-qmp.c | 4 ----
> include/hw/virtio/vhost.h | 7 -------
> qapi/virtio.json | 3 ---
> 10 files changed, 22 insertions(+), 41 deletions(-)
>
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index c0cc5f6942..de7a810c93 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -348,7 +348,6 @@ static int vhost_user_blk_connect(DeviceState *dev, Error
> **errp)
> s->dev.nvqs = s->num_queues;
> s->dev.vqs = s->vhost_vqs;
> s->dev.vq_index = 0;
> - s->dev.backend_features = 0;
>
> vhost_dev_set_config_notifier(&s->dev, &blk_ops);
>
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index 25e9f1fd24..fda90e231e 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -52,8 +52,14 @@ int vhost_net_set_config(struct vhost_net *net, const
> uint8_t *data,
>
> void vhost_net_ack_features_ex(struct vhost_net *net, const uint64_t
> *features)
> {
> - virtio_features_copy(net->dev.acked_features_ex,
> - net->dev.backend_features_ex);
> + virtio_features_clear(net->dev.acked_features_ex);
> + if (net->backend == -1) {
> + net->dev.acked_features =
> + net->dev.features & (1ULL << VHOST_USER_F_PROTOCOL_FEATURES);
> + } else if (!qemu_has_vnet_hdr(net->nc)) {
> + net->dev.acked_features = 1ULL << VHOST_NET_F_VIRTIO_NET_HDR;
> + }
> +
> vhost_ack_features_ex(&net->dev, net->feature_bits, features);
> }
>
> @@ -258,12 +264,9 @@ struct vhost_net *vhost_net_init(VhostNetOptions
> *options)
> if (r < 0) {
> goto fail;
> }
> - net->dev.backend_features = qemu_has_vnet_hdr(options->net_backend)
> - ? 0 : (1ULL << VHOST_NET_F_VIRTIO_NET_HDR);
> net->backend = r;
> net->dev.protocol_features = 0;
> } else {
> - virtio_features_clear(net->dev.backend_features_ex);
> net->dev.protocol_features = 0;
> net->backend = -1;
>
> @@ -284,13 +287,12 @@ struct vhost_net *vhost_net_init(VhostNetOptions
> *options)
> net->dev.features &= ~(1ULL << VIRTIO_NET_F_MRG_RXBUF);
> }
>
> - if (virtio_features_andnot(missing_features,
> - net->dev.backend_features_ex,
> - net->dev.features_ex)) {
> - fprintf(stderr, "vhost lacks feature mask 0x" VIRTIO_FEATURES_FMT
> - " for backend\n", VIRTIO_FEATURES_PR(missing_features));
> - goto fail;
> - }
> + if (!qemu_has_vnet_hdr(options->net_backend) &&
> + (~net->dev.features & (1ULL << VHOST_NET_F_VIRTIO_NET_HDR))) {
> + fprintf(stderr, "vhost lacks feature mask 0x%llx for backend\n",
> + ~net->dev.features & (1ULL <<
> VHOST_NET_F_VIRTIO_NET_HDR));
> + goto fail;
> + }
> }
>
> /* Set sane init value. Override when guest acks. */
> diff --git a/hw/scsi/vhost-scsi.c b/hw/scsi/vhost-scsi.c
> index cdf405b0f8..d694a25fe2 100644
> --- a/hw/scsi/vhost-scsi.c
> +++ b/hw/scsi/vhost-scsi.c
> @@ -276,7 +276,6 @@ static void vhost_scsi_realize(DeviceState *dev, Error
> **errp)
> vqs = g_new0(struct vhost_virtqueue, vsc->dev.nvqs);
> vsc->dev.vqs = vqs;
> vsc->dev.vq_index = 0;
> - vsc->dev.backend_features = 0;
>
> ret = vhost_dev_init(&vsc->dev, (void *)(uintptr_t)vhostfd,
> VHOST_BACKEND_TYPE_KERNEL, 0, errp);
> diff --git a/hw/scsi/vhost-user-scsi.c b/hw/scsi/vhost-user-scsi.c
> index 25f2d894e7..0c80a271d8 100644
> --- a/hw/scsi/vhost-user-scsi.c
> +++ b/hw/scsi/vhost-user-scsi.c
> @@ -159,7 +159,6 @@ static int vhost_user_scsi_connect(DeviceState *dev,
> Error **errp)
> vsc->dev.nvqs = VIRTIO_SCSI_VQ_NUM_FIXED + vs->conf.num_queues;
> vsc->dev.vqs = s->vhost_vqs;
> vsc->dev.vq_index = 0;
> - vsc->dev.backend_features = 0;
>
> ret = vhost_dev_init(&vsc->dev, &s->vhost_user, VHOST_BACKEND_TYPE_USER,
> 0,
> errp);
> diff --git a/hw/virtio/vdpa-dev.c b/hw/virtio/vdpa-dev.c
> index 4a7b970976..efd9f68420 100644
> --- a/hw/virtio/vdpa-dev.c
> +++ b/hw/virtio/vdpa-dev.c
> @@ -104,7 +104,6 @@ static void vhost_vdpa_device_realize(DeviceState *dev,
> Error **errp)
> v->dev.vqs = vqs;
> v->dev.vq_index = 0;
> v->dev.vq_index_end = v->dev.nvqs;
> - v->dev.backend_features = 0;
> v->started = false;
>
> ret = vhost_vdpa_get_iova_range(v->vhostfd, &iova_range);
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index f367ce06ce..b80174f489 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1448,14 +1448,15 @@ static int vhost_user_set_features(struct vhost_dev
> *dev,
> int ret;
>
> /*
> - * We need to include any extra backend only feature bits that
> - * might be needed by our device. Currently this includes the
> - * VHOST_USER_F_PROTOCOL_FEATURES bit for enabling protocol
> - * features.
> + * Don't lose VHOST_USER_F_PROTOCOL_FEATURES, which is vhost-user
> + * specific.
> */
> - ret = vhost_user_set_u64(dev, VHOST_USER_SET_FEATURES,
> - features | dev->backend_features,
> - log_enabled);
> + if (virtio_has_feature(dev->features, VHOST_USER_F_PROTOCOL_FEATURES)) {
> + features |= 1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
> + }
> +
> + ret = vhost_user_set_u64(dev, VHOST_USER_SET_FEATURES, features,
> + log_enabled);
>
> if (virtio_has_feature(dev->protocol_features,
> VHOST_USER_PROTOCOL_F_STATUS)) {
> @@ -2189,8 +2190,6 @@ static int vhost_user_backend_init(struct vhost_dev
> *dev, void *opaque,
> (dev->config_ops && dev->config_ops->vhost_dev_config_notifier);
> uint64_t protocol_features;
>
> - dev->backend_features |= 1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
> -
> err = vhost_user_get_u64(dev, VHOST_USER_GET_PROTOCOL_FEATURES,
> &protocol_features);
> if (err < 0) {
> diff --git a/hw/virtio/virtio-hmp-cmds.c b/hw/virtio/virtio-hmp-cmds.c
> index 1daae482d3..4bf9a3109d 100644
> --- a/hw/virtio/virtio-hmp-cmds.c
> +++ b/hw/virtio/virtio-hmp-cmds.c
> @@ -176,8 +176,6 @@ void hmp_virtio_status(Monitor *mon, const QDict *qdict)
> hmp_virtio_dump_features(mon, s->vhost_dev->features);
> monitor_printf(mon, " Acked features:\n");
> hmp_virtio_dump_features(mon, s->vhost_dev->acked_features);
> - monitor_printf(mon, " Backend features:\n");
> - hmp_virtio_dump_features(mon, s->vhost_dev->backend_features);
> monitor_printf(mon, " Protocol features:\n");
> hmp_virtio_dump_protocols(mon, s->vhost_dev->protocol_features);
> }
> diff --git a/hw/virtio/virtio-qmp.c b/hw/virtio/virtio-qmp.c
> index b338344c6c..dd1a38e792 100644
> --- a/hw/virtio/virtio-qmp.c
> +++ b/hw/virtio/virtio-qmp.c
> @@ -780,8 +780,6 @@ VirtioStatus *qmp_x_query_virtio_status(const char *path,
> Error **errp)
> vdev->guest_features_ex);
> status->host_features = qmp_decode_features(vdev->device_id,
> vdev->host_features_ex);
> - status->backend_features = qmp_decode_features(vdev->device_id,
> - vdev->backend_features_ex);
>
> switch (vdev->device_endian) {
> case VIRTIO_DEVICE_ENDIAN_LITTLE:
> @@ -822,8 +820,6 @@ VirtioStatus *qmp_x_query_virtio_status(const char *path,
> Error **errp)
> qmp_decode_features(vdev->device_id, hdev->features_ex);
> status->vhost_dev->acked_features =
> qmp_decode_features(vdev->device_id, hdev->acked_features_ex);
> - status->vhost_dev->backend_features =
> - qmp_decode_features(vdev->device_id, hdev->backend_features_ex);
>
> status->vhost_dev->protocol_features =
> qmp_decode_protocols(hdev->protocol_features);
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index 1ee639dd7e..3e69e47833 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -100,16 +100,9 @@ struct vhost_dev {
> *
> * @features: available features provided by the backend
> * @acked_features: final negotiated features with front-end driver
> - *
> - * @backend_features: this is used in a couple of places to either
> - * store VHOST_USER_F_PROTOCOL_FEATURES to apply to
> - * VHOST_USER_SET_FEATURES or VHOST_NET_F_VIRTIO_NET_HDR. Its
> - * future use should be discouraged and the variable retired as
> - * its easy to confuse with the VirtIO backend_features.
> */
> VIRTIO_DECLARE_FEATURES(features);
> VIRTIO_DECLARE_FEATURES(acked_features);
> - VIRTIO_DECLARE_FEATURES(backend_features);
>
> /**
> * @protocol_features: is the vhost-user only feature set by
> diff --git a/qapi/virtio.json b/qapi/virtio.json
> index 05295ab665..b995a5bb6d 100644
> --- a/qapi/virtio.json
> +++ b/qapi/virtio.json
> @@ -85,8 +85,6 @@
> #
> # @acked-features: vhost_dev acked_features
> #
> -# @backend-features: vhost_dev backend_features
> -#
> # @protocol-features: vhost_dev protocol_features
> #
> # @max-queues: vhost_dev max_queues
> @@ -106,7 +104,6 @@
> 'vq-index': 'int',
> 'features': 'VirtioDeviceFeatures',
> 'acked-features': 'VirtioDeviceFeatures',
> - 'backend-features': 'VirtioDeviceFeatures',
> 'protocol-features': 'VhostDeviceProtocols',
> 'max-queues': 'uint64',
> 'backend-cap': 'uint64',
> --
> 2.48.1
>
