[PATCH v2 0/2] virtio: remove unnecessary host_features in ->get_features()

[PATCH v2 0/2] virtio: remove unnecessary host_features in ->get_features()

[Stefan Hajnoczi]

v2:
- Document vdv->get_features() callback [Cornelia]

The vdc->get_features() callbacks are a little inconsistent in how they use
vdev->host_features. This is because the function's behavior changed over time.
Clean things up.

Stefan Hajnoczi (2):
  virtio: document vdc->get_features() callback
  virtio: remove unnecessary host_features in ->get_features()

 include/hw/virtio/virtio.h  | 20 ++++++++++++++++++++
 hw/block/virtio-blk.c       |  3 ---
 hw/char/virtio-serial-bus.c |  1 -
 hw/net/virtio-net.c         |  3 ---
 hw/scsi/vhost-scsi-common.c |  3 ---
 hw/scsi/virtio-scsi.c       |  4 ----
 hw/virtio/virtio-balloon.c  |  2 --
 7 files changed, 20 insertions(+), 16 deletions(-)

-- 
2.37.1

[PATCH v2 1/2] virtio: document vdc->get_features() callback

[Stefan Hajnoczi]

Suggested-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/hw/virtio/virtio.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index db1c0ddf6b..8d27fe1824 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -120,9 +120,29 @@ struct VirtioDeviceClass {
     /* This is what a VirtioDevice must implement */
     DeviceRealize realize;
     DeviceUnrealize unrealize;
+
+    /**
+     * get_features:
+     * @vdev: the VirtIODevice
+     * @requested_features: existing device feature bits from
+     *                      vdev->host_features
+     * @errp: pointer to error object
+     *
+     * Get the device feature bits.
+     *
+     * The ->get_features() function typically sets always-on device feature
+     * bits as well as conditional feature bits that require some logic to
+     * compute.
+     *
+     * Device feature bits can also be set in vdev->host_features before this
+     * function is called using DEFINE_PROP_BIT64() qdev properties.
+     *
+     * Returns: the final device feature bits to store in vdev->host_features.
+     */
     uint64_t (*get_features)(VirtIODevice *vdev,
                              uint64_t requested_features,
                              Error **errp);
+
     uint64_t (*bad_features)(VirtIODevice *vdev);
     void (*set_features)(VirtIODevice *vdev, uint64_t val);
     int (*validate_features)(VirtIODevice *vdev);
-- 
2.37.1

[PATCH v2 2/2] virtio: remove unnecessary host_features in ->get_features()

[Stefan Hajnoczi]

Since at least commit 6b8f1020540c27246277377aa2c3331ad2bfb160 ("virtio:
move host_features") the ->get_features() function has been called with
host_features as an argument.

Some devices manually add host_features in ->get_features() although the
features argument already contains host_features. Make all devices
consistent by dropping the unnecessary code.

Cc: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/block/virtio-blk.c       | 3 ---
 hw/char/virtio-serial-bus.c | 1 -
 hw/net/virtio-net.c         | 3 ---
 hw/scsi/vhost-scsi-common.c | 3 ---
 hw/scsi/virtio-scsi.c       | 4 ----
 hw/virtio/virtio-balloon.c  | 2 --
 6 files changed, 16 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index e9ba752f6b..429aedcf2b 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -996,9 +996,6 @@ static uint64_t virtio_blk_get_features(VirtIODevice *vdev, uint64_t features,
 {
     VirtIOBlock *s = VIRTIO_BLK(vdev);
 
-    /* Firstly sync all virtio-blk possible supported features */
-    features |= s->host_features;
-
     virtio_add_feature(&features, VIRTIO_BLK_F_SEG_MAX);
     virtio_add_feature(&features, VIRTIO_BLK_F_GEOMETRY);
     virtio_add_feature(&features, VIRTIO_BLK_F_TOPOLOGY);
diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
index 7d4601cb5d..1414fb85ae 100644
--- a/hw/char/virtio-serial-bus.c
+++ b/hw/char/virtio-serial-bus.c
@@ -557,7 +557,6 @@ static uint64_t get_features(VirtIODevice *vdev, uint64_t features,
 
     vser = VIRTIO_SERIAL(vdev);
 
-    features |= vser->host_features;
     if (vser->bus.max_nr_ports > 1) {
         virtio_add_feature(&features, VIRTIO_CONSOLE_F_MULTIPORT);
     }
diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index dd0d056fde..8ecdc1cd83 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -715,9 +715,6 @@ static uint64_t virtio_net_get_features(VirtIODevice *vdev, uint64_t features,
     VirtIONet *n = VIRTIO_NET(vdev);
     NetClientState *nc = qemu_get_queue(n->nic);
 
-    /* Firstly sync all virtio-net possible supported features */
-    features |= n->host_features;
-
     virtio_add_feature(&features, VIRTIO_NET_F_MAC);
 
     if (!peer_has_vnet_hdr(n)) {
diff --git a/hw/scsi/vhost-scsi-common.c b/hw/scsi/vhost-scsi-common.c
index 767f827e55..8b26f90aa1 100644
--- a/hw/scsi/vhost-scsi-common.c
+++ b/hw/scsi/vhost-scsi-common.c
@@ -124,9 +124,6 @@ uint64_t vhost_scsi_common_get_features(VirtIODevice *vdev, uint64_t features,
 {
     VHostSCSICommon *vsc = VHOST_SCSI_COMMON(vdev);
 
-    /* Turn on predefined features supported by this device */
-    features |= vsc->host_features;
-
     return vhost_get_features(&vsc->dev, vsc->feature_bits, features);
 }
 
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 4141dddd51..f754611dfe 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -816,10 +816,6 @@ static uint64_t virtio_scsi_get_features(VirtIODevice *vdev,
                                          uint64_t requested_features,
                                          Error **errp)
 {
-    VirtIOSCSI *s = VIRTIO_SCSI(vdev);
-
-    /* Firstly sync all virtio-scsi possible supported features */
-    requested_features |= s->host_features;
     return requested_features;
 }
 
diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index 73ac5eb675..0e9ca71b15 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -796,8 +796,6 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
 static uint64_t virtio_balloon_get_features(VirtIODevice *vdev, uint64_t f,
                                             Error **errp)
 {
-    VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
-    f |= dev->host_features;
     virtio_add_feature(&f, VIRTIO_BALLOON_F_STATS_VQ);
 
     return f;
-- 
2.37.1

Re: [PATCH v2 1/2] virtio: document vdc->get_features() callback

[Cornelia Huck]

On Wed, Aug 03 2022, Stefan Hajnoczi <stefanha@redhat.com> wrote:

> Suggested-by: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/hw/virtio/virtio.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index db1c0ddf6b..8d27fe1824 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -120,9 +120,29 @@ struct VirtioDeviceClass {
>      /* This is what a VirtioDevice must implement */
>      DeviceRealize realize;
>      DeviceUnrealize unrealize;
> +
> +    /**
> +     * get_features:
> +     * @vdev: the VirtIODevice
> +     * @requested_features: existing device feature bits from
> +     *                      vdev->host_features
> +     * @errp: pointer to error object
> +     *
> +     * Get the device feature bits.
> +     *
> +     * The ->get_features() function typically sets always-on device feature
> +     * bits as well as conditional feature bits that require some logic to
> +     * compute.
> +     *
> +     * Device feature bits can also be set in vdev->host_features before this
> +     * function is called using DEFINE_PROP_BIT64() qdev properties.
> +     *
> +     * Returns: the final device feature bits to store in vdev->host_features.
> +     */

Not sure if we want to go full function doc for features, as none of the
other callbacks have it...

I thought about something like

"Called with vdev->host_features in requested_features. Returns device
feature bits to be stored in vdev->host_features after factoring in
device-specific feature bits."

The important part IMHO is that requested_features contains
vdev->host_features, so no need to merge them in.

>      uint64_t (*get_features)(VirtIODevice *vdev,
>                               uint64_t requested_features,
>                               Error **errp);
> +
>      uint64_t (*bad_features)(VirtIODevice *vdev);
>      void (*set_features)(VirtIODevice *vdev, uint64_t val);
>      int (*validate_features)(VirtIODevice *vdev);

Re: [PATCH v2 2/2] virtio: remove unnecessary host_features in ->get_features()

[Cornelia Huck]

On Wed, Aug 03 2022, Stefan Hajnoczi <stefanha@redhat.com> wrote:

> Since at least commit 6b8f1020540c27246277377aa2c3331ad2bfb160 ("virtio:
> move host_features") the ->get_features() function has been called with
> host_features as an argument.
>
> Some devices manually add host_features in ->get_features() although the
> features argument already contains host_features. Make all devices
> consistent by dropping the unnecessary code.
>
> Cc: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  hw/block/virtio-blk.c       | 3 ---
>  hw/char/virtio-serial-bus.c | 1 -
>  hw/net/virtio-net.c         | 3 ---
>  hw/scsi/vhost-scsi-common.c | 3 ---
>  hw/scsi/virtio-scsi.c       | 4 ----
>  hw/virtio/virtio-balloon.c  | 2 --
>  6 files changed, 16 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>