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

[Laurent Vivier <lvivier@redhat.com>]

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.

> 
> 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?

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
>