Re: [dpdk-dev] [PATCH v3 1/4] vhost: prevent features to be changed while device is running

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

On 12/07/2017 09:08 AM, Tiwei Bie wrote:
> On Wed, Dec 06, 2017 at 10:20:45AM +0100, Maxime Coquelin wrote:
>> As section 2.2 of the Virtio spec states about features
>> negotiation:
>> "During device initialization, the driver reads this and tells
>> the device the subset that it accepts. The only way to
>> renegotiate is to reset the device."
>>
>> This patch implements a check to prevent illegal features change
>> while the device is running.
>>
>> One exception is the VHOST_F_LOG_ALL feature bit, which is enabled
>> when live-migration is initiated. But this feature is not negotiated
>> with the Virtio driver, but directly with the Vhost master.
>>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>> ---
>>   lib/librte_vhost/vhost_user.c | 17 ++++++++++++++++-
>>   1 file changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
>> index f4c7ce462..2d86c0ca8 100644
>> --- a/lib/librte_vhost/vhost_user.c
>> +++ b/lib/librte_vhost/vhost_user.c
>> @@ -183,7 +183,22 @@ vhost_user_set_features(struct virtio_net *dev, uint64_t features)
>>   		return -1;
>>   	}
>>   
>> -	if ((dev->flags & VIRTIO_DEV_RUNNING) && dev->features != features) {
>> +	if (dev->features == features)
>> +		return 0;
>> +
> 
> We couldn't return directly when dev->features == features.
> Otherwise, if the features provided by virtio driver is 0,
> dev->vhost_hlen won't get a chance to be initialized.

Good catch.

Either we do :
if ((dev->features == features) && dev->vhost_len)
     return 0;

Or we could initialize dev->vhost_len to sizeof(struct virtio_net_hdr)
at alloc time.

I prefer the former, what do you think?

Thanks,
Maxime

> Best regards,
> Tiwei Bie
>