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

Jason Wang
2026-03-13T06:05:11Z

On Thu, Mar 12, 2026 at 2:24 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Thu, Mar 12, 2026 at 4:58 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > 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);
> > }
> >
>
> I'm not sure I follow you here.
>
> We can't do better than breaking the device because the function
> returns no error state, and the caller does not expect an error code.
> Do you mean we can't depend on vduse_dev_msg_sync to call
> vduse_dev_broken(dev) by itself?

I think I meant, reset seems to be more heavyweight than suspend.

So if reset can fail, I don't see reason ot break device only for
suspend failure.

Thanks

>
> > > + 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
> >
>