Re: [PATCH v3 3/3] vduse: add F_QUEUE_READY feature

[Jason Wang <jasowang@redhat.com>]

On Wed, Mar 11, 2026 at 3:08 AM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> Add the VDUSE_F_QUEUE_READY feature flag. This allows the kernel module
> to explicitly signal userspace when a specific virtqueue has been
> enabled.
>
> In scenarios like Live Migration of VirtIO net devices, the dataplane
> starts after the control virtqueue allowing QEMU to apply configuration
> in the destination device.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
> v2:
> * Fix comment of vduse_dev_request.vq_ready
> * Set vq_ready before sending the message to the VDUSE userland
>   instance, avoiding the need for SMP sync after receiving the message.
> ---
>  drivers/vdpa/vdpa_user/vduse_dev.c | 28 +++++++++++++++++++++++++++-
>  include/uapi/linux/vduse.h         | 18 ++++++++++++++++++
>  2 files changed, 45 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> index 17e0358d3a68..4f642b95a7cb 100644
> --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> @@ -9,6 +9,7 @@
>   */
>
>  #include "linux/virtio_net.h"
> +#include <linux/bits.h>
>  #include <linux/cleanup.h>
>  #include <linux/init.h>
>  #include <linux/module.h>
> @@ -53,7 +54,7 @@
>  #define IRQ_UNBOUND -1
>
>  /* Supported VDUSE features */
> -static const uint64_t vduse_features;
> +static const uint64_t vduse_features = BIT_U64(VDUSE_F_QUEUE_READY);
>
>  /*
>   * VDUSE instance have not asked the vduse API version, so assume 0.
> @@ -120,6 +121,7 @@ struct vduse_dev {
>         char *name;
>         struct mutex lock;
>         spinlock_t msg_lock;
> +       u64 vduse_features;
>         u64 msg_unique;
>         u32 msg_timeout;
>         wait_queue_head_t waitq;
> @@ -601,8 +603,29 @@ static void vduse_vdpa_set_vq_ready(struct vdpa_device *vdpa,
>  {
>         struct vduse_dev *dev = vdpa_to_vduse(vdpa);
>         struct vduse_virtqueue *vq = dev->vqs[idx];
> +       struct vduse_dev_msg msg = { 0 };
> +       int r;
>
>         vq->ready = ready;
> +
> +       if (!(dev->vduse_features & BIT_U64(VDUSE_F_QUEUE_READY)))
> +               return;
> +
> +       msg.req.type = VDUSE_SET_VQ_READY;
> +       msg.req.vq_ready.num = idx;
> +       msg.req.vq_ready.ready = !!ready;
> +
> +       r = vduse_dev_msg_sync(dev, &msg);
> +
> +       if (r < 0) {
> +               dev_dbg(&vdpa->dev, "device refuses to set vq %u ready %u",
> +                       idx, ready);
> +
> +               /* We can't do better than break the device in this case */

It's better to explain why we can't depend on vduse_dev_msg_sync() here.

For example it did:

        if (unlikely(dev->broken))
                return -EIO;

        init_waitqueue_head(&msg->waitq);
        spin_lock(&dev->msg_lock);
        if (unlikely(dev->broken)) {
                spin_unlock(&dev->msg_lock);
                return -EIO;
        }

and

        if (!msg->completed) {
                list_del(&msg->list);
                msg->resp.result = VDUSE_REQ_RESULT_FAILED;
                /* Mark the device as malfunction when there is a timeout */
                if (!ret)
                        vduse_dev_broken(dev);
        }

> +               spin_lock(&dev->msg_lock);
> +               vduse_dev_broken(dev);
> +               spin_unlock(&dev->msg_lock);
> +       }
>  }
>
>  static bool vduse_vdpa_get_vq_ready(struct vdpa_device *vdpa, u16 idx)
> @@ -2078,6 +2101,9 @@ static int vduse_create_dev(struct vduse_dev_config *config,
>         dev->device_features = config->features;
>         dev->device_id = config->device_id;
>         dev->vendor_id = config->vendor_id;
> +       dev->vduse_features = config->vduse_features;
> +       dev_dbg(vduse_ctrl_dev, "Creating device %s with features 0x%llx",
> +               config->name, config->vduse_features);
>
>         dev->nas = (dev->api_version < VDUSE_API_VERSION_1) ? 1 : config->nas;
>         dev->as = kcalloc(dev->nas, sizeof(dev->as[0]), GFP_KERNEL);
> diff --git a/include/uapi/linux/vduse.h b/include/uapi/linux/vduse.h
> index e9b5f32a452d..7324faea5df4 100644
> --- a/include/uapi/linux/vduse.h
> +++ b/include/uapi/linux/vduse.h
> @@ -14,6 +14,9 @@
>
>  #define VDUSE_API_VERSION_1    1
>
> +/* The VDUSE instance expects a request for vq ready */
> +#define VDUSE_F_QUEUE_READY    0
> +
>  /*
>   * Get the version of VDUSE API that kernel supported (VDUSE_API_VERSION).
>   * This is used for future extension.
> @@ -330,6 +333,7 @@ enum vduse_req_type {
>         VDUSE_SET_STATUS,
>         VDUSE_UPDATE_IOTLB,
>         VDUSE_SET_VQ_GROUP_ASID,
> +       VDUSE_SET_VQ_READY,
>  };
>
>  /**
> @@ -377,6 +381,15 @@ struct vduse_iova_range_v2 {
>         __u32 padding;
>  };
>
> +/**
> + * struct vduse_vq_ready - Virtqueue ready request message
> + * @num: Virtqueue number
> + */
> +struct vduse_vq_ready {
> +       __u32 num;
> +       __u32 ready;
> +};
> +
>  /**
>   * struct vduse_dev_request - control request
>   * @type: request type
> @@ -387,6 +400,7 @@ struct vduse_iova_range_v2 {
>   * @iova: IOVA range for updating
>   * @iova_v2: IOVA range for updating if API_VERSION >= 1
>   * @vq_group_asid: ASID of a virtqueue group
> + * @vq_ready: Virtqueue ready request
>   * @padding: padding
>   *
>   * Structure used by read(2) on /dev/vduse/$NAME.
> @@ -404,6 +418,10 @@ struct vduse_dev_request {
>                  */
>                 struct vduse_iova_range_v2 iova_v2;
>                 struct vduse_vq_group_asid vq_group_asid;
> +
> +               /* Only if VDUSE_F_QUEUE_READY is negotiated */
> +               struct vduse_vq_ready vq_ready;
> +
>                 __u32 padding[32];
>         };
>  };
> --
> 2.53.0
>

Thanks