Re: [PATCH v2 4/7] vhost: involve device backends in feature negotiation

[Jason Wang <jasowang@redhat.com>]

On 2020/6/10 上午2:07, Michael S. Tsirkin wrote:
>> +/*
>> + * Default vhost_get_features() feature bits for existing device types that do
>> + * not define their own.
>> + *
>> + * This is a workaround for existing device types, do not use this in new vhost
>> + * device types. Explicitly define a list of feature bits instead.
>> + *
>> + * The following feature bits are excluded because libvhost-user device
>> + * backends did not advertise them for a long time. Therefore we cannot detect
>> + * their presence. Instead we assume they are always supported by the device
>> + * backend:
>> + * VIRTIO_F_NOTIFY_ON_EMPTY
>> + * VIRTIO_F_ANY_LAYOUT
>> + * VIRTIO_F_VERSION_1
>> + * VIRTIO_RING_F_INDIRECT_DESC
>> + * VIRTIO_RING_F_EVENT_IDX
> Weird. I remember that it's common for vhost-user not to set
> VIRTIO_RING_F_INDIRECT_DESC - they have huge queues so
> don't need it and inline descriptors give them better
> performance.
>
> So what's going on here?


I guess one reason is to support live migration between vhost-user and 
vhost-net.

Thanks