Re: [dpdk-dev] [PATCH v4 4/4] vhost: destroy unused virtqueues when multiqueue not negotiated

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 12/13/2017 04:16 AM, Tiwei Bie wrote:
> On Mon, Dec 11, 2017 at 04:15:03PM +0100, Maxime Coquelin wrote:
>> QEMU sends VHOST_USER_SET_VRING_CALL requests for all queues
>> declared in QEMU command line before the guest is started.
>> It has the effect in DPDK vhost-user backend to allocate vrings
>> for all queues declared by QEMU.
>>
>> If the first driver being used does not support multiqueue,
>> the device never changes to VIRTIO_DEV_RUNNING state as only
>> the first queue pair is initialized. One driver impacted by
>> this bug is virtio-net's iPXE driver which does not support
>> VIRTIO_NET_F_MQ feature.
>>
>> It is safe to destroy unused virtqueues in SET_FEATURES request
>> handler, as it is ensured the device is not in running state
>> at this stage, so virtqueues aren't being processed.
>>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>> ---
>>   lib/librte_vhost/vhost_user.c | 18 ++++++++++++++++++
>>   1 file changed, 18 insertions(+)
>>
>> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
>> index 471b1612c..d5ca1ac90 100644
>> --- a/lib/librte_vhost/vhost_user.c
>> +++ b/lib/librte_vhost/vhost_user.c
>> @@ -216,6 +216,24 @@ vhost_user_set_features(struct virtio_net *dev, uint64_t features)
>>   		(dev->features & (1 << VIRTIO_NET_F_MRG_RXBUF)) ? "on" : "off",
>>   		(dev->features & (1ULL << VIRTIO_F_VERSION_1)) ? "on" : "off");
>>   
>> +	if (!(dev->features & (1ULL << VIRTIO_NET_F_MQ))) {
>> +		/*
>> +		 * Remove all but first queue pair if MQ hasn't been
>> +		 * negotiated. This is safe because the device is not
>> +		 * running at this stage.
>> +		 */
>> +		while (dev->nr_vring > 2) {
>> +			struct vhost_virtqueue *vq;
>> +
>> +			vq = dev->virtqueue[--dev->nr_vring];
>> +			if (!vq)
>> +				continue;
>> +
>> +			cleanup_vq(vq, 1);
>> +			free_vq(vq);
> 
> Hi,
> 
> Sorry, I didn't look into this patch in last version.

Don't be sorry, thanks for catching this bug.

> The freed dev->virtqueue[$idx] also needs to be zeroed.
> Otherwise, it won't be allocated in the future due to the
> below check in vhost_user_check_and_alloc_queue_pair(),
> and the freed memory will be used again.
> 
> /*
>   * Allocate a queue pair if it hasn't been allocated yet
>   */
> static int
> vhost_user_check_and_alloc_queue_pair(struct virtio_net *dev, VhostUserMsg *msg)
> {
>          ........
> 
> 	if (dev->virtqueue[vring_idx])
> 		return 0;
> 
> 	return alloc_vring_queue(dev, vring_idx);
> }

You are right, I'll post v5 setting dev->virtqueue[$idx] to NULL after
free_vq() call.

Thanks for the review,
Maxime

> Best regards,
> Tiwei Bie
> 
>> +		}
>> +	}
>> +
>>   	return 0;
>>   }
>>   
>> -- 
>> 2.14.3
>>