Re: [PATCH v2] vhost-user-snd: correct the calculation of config_size

[Stefano Garzarella]

On Mon, Feb 17, 2025 at 02:12:55PM +0100, Matias Ezequiel Vara Larsen wrote:
Use virtio_get_config_size() rather than sizeof(struct
virtio_snd_config) for the config_size in the vhost-user-snd frontend.
The frontend shall rely on device features for the size of the device
configuration space. The presence of `controls` in the config space
depends on VIRTIO_SND_F_CTLS according to the specification (v1.3):
`
5.14.4 Device Configuration Layout
...
controls
(driver-read-only) indicates a total number of all available control
elements if VIRTIO_SND_F_CTLS has been negotiated.
`
This fixes an issue introduced by commit ab0c7fb2 ("linux-headers:
update to current kvm/next") in which the optional field `controls` is
added to the virtio_snd_config structure. This breaks vhost-user-device
backends that do not implement the `controls` field.

Fixes: ab0c7fb22b ("linux-headers: update to current kvm/next")

If we want to backport this on stable branches, better to add:

Cc: qemu-sta...@nongnu.org

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2805
Reviewed-by: Philippe Mathieu-Daudé <phi...@linaro.org>
Suggested-by: Stefano Garzarella <sgarz...@redhat.com>
Signed-off-by: Matias Ezequiel Vara Larsen <mvara...@redhat.com>
---
Changes in v2:
- Addressed comments from Stefano Garzarella about commit msg and the
  property name.

Thanks for that! LGTM, I also tested with old vhost-device-sound and it 
works well!

Reviewed-by: Stefano Garzarella <sgarz...@redhat.com>

---
hw/virtio/vhost-user-snd.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost-user-snd.c b/hw/virtio/vhost-user-snd.c
index 8610370af8..b414c75c06 100644
--- a/hw/virtio/vhost-user-snd.c
+++ b/hw/virtio/vhost-user-snd.c
@@ -16,6 +16,18 @@
#include "standard-headers/linux/virtio_ids.h"
#include "standard-headers/linux/virtio_snd.h"

+static const VirtIOFeature feature_sizes[] = {
+    {.flags = 1ULL << VIRTIO_SND_F_CTLS,
+    .end = endof(struct virtio_snd_config, controls)},
+    {}
+};
+
+static const VirtIOConfigSizeParams cfg_size_params = {
+    .min_size = endof(struct virtio_snd_config, chmaps),
+    .max_size = sizeof(struct virtio_snd_config),
+    .feature_sizes = feature_sizes
+};
+
static const VMStateDescription vu_snd_vmstate = {
    .name = "vhost-user-snd",
    .unmigratable = 1,
@@ -23,16 +35,20 @@ static const VMStateDescription vu_snd_vmstate = {

static const Property vsnd_properties[] = {
    DEFINE_PROP_CHR("chardev", VHostUserBase, chardev),
+    DEFINE_PROP_BIT64("controls", VHostUserBase,
+                      parent_obj.host_features, VIRTIO_SND_F_CTLS, false),
};

static void vu_snd_base_realize(DeviceState *dev, Error **errp)
{
    VHostUserBase *vub = VHOST_USER_BASE(dev);
    VHostUserBaseClass *vubs = VHOST_USER_BASE_GET_CLASS(dev);
+    VirtIODevice *vdev = &vub->parent_obj;

    vub->virtio_id = VIRTIO_ID_SOUND;
    vub->num_vqs = 4;
-    vub->config_size = sizeof(struct virtio_snd_config);
+    vub->config_size = virtio_get_config_size(&cfg_size_params,
+                                              vdev->host_features);
    vub->vq_size = 64;

    vubs->parent_realize(dev, errp);
--
2.42.0