>
> On Sat, Sep 19, 2026 at 09:17:44PM +0800, Jia Jia wrote:
> > vhost-scsi keeps the response and bounce-buffer iovecs until the backend
> > completes the command. If userspace changes the vring addresses while the
> > command is in flight, the completion data is copied through the old iovecs
> > while vhost_add_used() updates the new used ring. The guest then no longer
> > sees the completion.
>
> I do not get what all this is trying to say. if you break it you get to
> keep both pieces. when is this a real problem?
>
Before running the test, I checked the virtio spec and vhost UAPI and
did not find an explicit rule
prohibiting changes to vring addresses while a backend is attached.
That is why I ran the directed test.
I see what you mean now. I'll drop this patch.
> >
> > Reject changes to the vring addresses while a backend is attached to the
> > queue. Keep accepting the current addresses so userspace can update
> > VHOST_VRING_F_LOG or log_guest_addr without stopping the backend. Queues
> > that have not been attached retain the existing setup behavior; userspace
> > must detach the backend before changing their addresses.
> >
> > Fixes: 057cbf49a1f0 ("tcm_vhost: Initial merge for vhost level target
> > fabric driver")
> > Signed-off-by: Jia Jia <[email protected]>
> > ---
> > drivers/vhost/vhost.c | 11 +++++++++++
> > 1 file changed, 11 insertions(+)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 44cac11b68d2..074590838de5 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2150,6 +2150,17 @@ static long vhost_vring_set_addr(struct vhost_dev *d,
> > a.flags & (0x1 << VHOST_VRING_F_LOG),
> > a.log_guest_addr))
> > return -EINVAL;
> > +
> > + /*
> > + * Commands may retain iovecs derived from the current ring
> > + * addresses. Keep the addresses unchanged while the backend
> > + * is attached.
> > + */
> > + if ((vq->desc || vq->avail || vq->used) &&
> > + (vq->desc != (void __user *)(unsigned
> > long)a.desc_user_addr ||
> > + vq->avail != (void __user *)(unsigned
> > long)a.avail_user_addr ||
> > + vq->used != (void __user *)(unsigned
> > long)a.used_user_addr))
> > + return -EBUSY;
> > }
> >
> > vq->log_used = !!(a.flags & (0x1 << VHOST_VRING_F_LOG));
>
