Re: [RFC PATCH] include/hw: attempt to document VirtIO feature variables (!DISCUSS!)

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, 21 Nov 2022 at 09:49, Alex Bennée <alex.bennee@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.bennee@linaro.org>
> Cc: Stefano Garzarella <sgarzare@redhat.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@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?

> +     *
> +     * 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?

> +     * 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.

> 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?

> +     */
>      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
>