Re: [Qemu-devel] [PATCH] vhost_net: do not expose MTU feature bit to kernel backend

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

On 05/22/2017 07:24 PM, Michael S. Tsirkin wrote:
> On Sat, May 20, 2017 at 10:06:58AM +0200, Maxime Coquelin wrote:
>> This patch removes MTU from the list of features supported by
>> the kernel backend, so that vhost kernel backend does not have
>> to advertise it to be negotiated.
>>
>> Cc: Aaron Conole <aconole@redhat.com>
>> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> 
> This will break cross-version migration when the kernel
> does not advertise it though. I'm afraid we'll need
> a compat flag.

Ok, you are right.
The difficulty with a compat flag is that vhost-net is not a driver,
so we cannot add it a new property to make it kernel-backend only.

For consistency, maybe we can add a new internal property to virtio-net
driver, to bypass backend negotiation for this feature for all backends?

I will propose a new version shortly.

Thanks,
Maxime
>> ---
>>   hw/net/vhost_net.c | 1 -
>>   1 file changed, 1 deletion(-)
>>
>> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
>> index 22874a9..bdd8633 100644
>> --- a/hw/net/vhost_net.c
>> +++ b/hw/net/vhost_net.c
>> @@ -51,7 +51,6 @@ static const int kernel_feature_bits[] = {
>>       VIRTIO_RING_F_EVENT_IDX,
>>       VIRTIO_NET_F_MRG_RXBUF,
>>       VIRTIO_F_VERSION_1,
>> -    VIRTIO_NET_F_MTU,
>>       VIRTIO_F_IOMMU_PLATFORM,
>>       VHOST_INVALID_FEATURE_BIT
>>   };
>> -- 
>> 2.9.3