Re: [PATCH 1/2] virtio: add a way to disable a queue

[Jason Wang]

在 2021/8/2 下午4:42, Laurent Vivier 写道:
> On 02/08/2021 06:50, Jason Wang wrote:
> > 在 2021/7/30 上午3:19, Laurent Vivier 写道:
> > > Add virtio_queue_disable()/virtio_queue_enable() to disable/enable a queue
> > > by setting vring.num to 0 (or num_default).
> > > This is needed to be able to disable a guest driver from the host side
> >
> > I suspect this won't work correclty for vhost.
> With my test it seems to work with vhost too.


So setting 0 will lead -EINVAL to be returned during 
VHOST_SET_VRING_NUM. I think qemu will warn the failure in this case.

What's more important, it's not guaranteed to work for the case of 
vhost-user or vhost-vDPA.


> > And I believe we should only do this after the per queue enabling/disabling is 
> > supported
> > by the spec.
> >
> > (only MMIO support that AFAIK)
> I don't want to modify the spec.
>
> I need something that works without modifying existing (old) drivers.
>
> The idea is to be able to disable the virtio-net kernel driver from QEMU if the 
> driver is
> too old (i.e. it doesn't support STANDBY feature).
>
> Setting vring.num to 0 forces the kernel driver to exit on error in the probe 
> function.
> It's what I want: the device is present but disabled (the driver is not loaded).
>
> Any other suggestion?


I think we should probably disable the device instead of doing it per 
virtqueue.

Thanks


> Thanks,
> Laurent
>
> > Thanks
> >
> >
> > > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > > ---
> > >    include/hw/virtio/virtio.h |  2 ++
> > >    hw/virtio/virtio.c         | 10 ++++++++++
> > >    2 files changed, 12 insertions(+)
> > >
> > > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > > index 8bab9cfb7507..6a3f71b4cd88 100644
> > > --- a/include/hw/virtio/virtio.h
> > > +++ b/include/hw/virtio/virtio.h
> > > @@ -251,6 +251,8 @@ void virtio_config_modern_writel(VirtIODevice *vdev,
> > >                                     uint32_t addr, uint32_t data);
> > >    void virtio_queue_set_addr(VirtIODevice *vdev, int n, hwaddr addr);
> > >    hwaddr virtio_queue_get_addr(VirtIODevice *vdev, int n);
> > > +void virtio_queue_enable(VirtIODevice *vdev, int n);
> > > +void virtio_queue_disable(VirtIODevice *vdev, int n);
> > >    void virtio_queue_set_num(VirtIODevice *vdev, int n, int num);
> > >    int virtio_queue_get_num(VirtIODevice *vdev, int n);
> > >    int virtio_queue_get_max_num(VirtIODevice *vdev, int n);
> > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > > index 874377f37a70..fa5228c1a2d6 100644
> > > --- a/hw/virtio/virtio.c
> > > +++ b/hw/virtio/virtio.c
> > > @@ -2244,6 +2244,16 @@ void virtio_queue_set_rings(VirtIODevice *vdev, int n, 
> > > hwaddr desc,
> > >        virtio_init_region_cache(vdev, n);
> > >    }
> > >    +void virtio_queue_disable(VirtIODevice *vdev, int n)
> > > +{
> > > +    vdev->vq[n].vring.num = 0;
> > > +}
> > > +
> > > +void virtio_queue_enable(VirtIODevice *vdev, int n)
> > > +{
> > > +    vdev->vq[n].vring.num = vdev->vq[n].vring.num_default;
> > > +}
> > > +
> > >    void virtio_queue_set_num(VirtIODevice *vdev, int n, int num)
> > >    {
> > >        /* Don't allow guest to flip queue between existent and