linux-kernel - Re: [PATCH 5/6] vduse: add F_QUEUE

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Message-ID: <CAJaqyWc8_B82n1a3JaBKKvhkWN=Rs93=kcfCU6e9RfzsPXdF5g@mail.gmail.com>
Date: Thu, 29 Jan 2026 07:26:02 +0100
From: Eugenio Perez Martin <eperezma@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: "Michael S . Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Cindy Lu <lulu@...hat.com>, Laurent Vivier <lvivier@...hat.com>, 
	Stefano Garzarella <sgarzare@...hat.com>, linux-kernel@...r.kernel.org, 
	Maxime Coquelin <mcoqueli@...hat.com>, Yongji Xie <xieyongji@...edance.com>, 
	virtualization@...ts.linux.dev
Subject: Re: [PATCH 5/6] vduse: add F_QUEUE_READY feature

On Thu, Jan 29, 2026 at 3:12 AM Jason Wang <jasowang@...hat.com> wrote:
>
> On Wed, Jan 28, 2026 at 8:45 PM Eugenio Pérez <eperezma@...hat.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@...hat.com>
> > ---
> >  drivers/vdpa/vdpa_user/vduse_dev.c | 28 +++++++++++++++++++++++++++-
> >  include/uapi/linux/vduse.h         | 19 +++++++++++++++++++
> >  2 files changed, 46 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> > index e7da69c2ad71..1d93b540db4d 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;
> > @@ -619,7 +621,30 @@ 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;
> > +
> > +       if (!(dev->vduse_features & BIT_U64(VDUSE_F_QUEUE_READY)))
> > +               goto out;
> > +
> > +       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 */
> > +               spin_lock(&dev->msg_lock);
> > +               vduse_dev_broken(dev);
>
> This has been done by vduse_dev_msg_sync().
>

This is done by msg_sync() when userland does not reply in a
timeframe, but not when userland replies with VDUSE_REQ_RESULT_FAILED.
Should I add a comment?