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

[Jason Wang]

在 2021/8/6 下午3:27, Laurent Vivier 写道:
> On 06/08/2021 08:25, Jason Wang wrote:
> > 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.
> I didn't see any error when I tried. I will check the code.
>
> > What's more important, it's not guaranteed to work for the case of vhost-user 
> > or vhost-vDPA.
> Perhaps we can target only the vhost host case, as this is used for failover 
> and usually
> the virtio-net device is backed by a bridge on same network as the VFIO device?


Probably not, it should be a general feature that can work for all types 
of virtio/vhost backends.


> > > > 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.
> I tried to use virtio_set_disabled() but it doesn't work.
> Perhaps it's too late when I call the function (I need to do that in
> virtio_net_set_features()). What I want is to prevent the load of the driver in 
> the guest
> kernel to hide the virtio-net device. Setting vring.num to 0 triggers an error 
> in the
> driver probe function and prevents the load of the driver.


How about fail the validate_features() in this case?

Thanks


> Thanks,
> Laurent