include/hw: attempt to document VirtIO feature variables

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.

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>

Message-Id: <20221123152134.179929-2-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

2 files changed, 40 insertions, 4 deletions

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 67a6807..1cafa0d 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -88,13 +88,32 @@ 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 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.
+     */
     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 */
+
+    /**
+     * @protocol_features: is the vhost-user only feature set by
+     * VHOST_USER_SET_PROTOCOL_FEATURES. Protocol features are only
+     * negotiated if VHOST_USER_F_PROTOCOL_FEATURES has been offered
+     * by the backend (see @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 acfd4df..24561e9 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 +106,20 @@ 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 has
+     * selected by writing to the feature register. Finally
+     * @backend_features represents everything supported by the
+     * backend (e.g. vhost) and could potentially be a subset of the
+     * total feature set offered by QEMU.
+     */
     uint64_t host_features;
+    uint64_t guest_features;
     uint64_t backend_features;
+
     size_t config_len;
     void *config;
     uint16_t config_vector;