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

[Jason Wang <jasowang@redhat.com>]

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
>