Re: [Qemu-devel] [PATCH] vhost-user: sync backend features

[Marcel Apfelbaum <marcel@redhat.com>]

On 07/29/2015 09:05 PM, Michael S. Tsirkin wrote:
> On Wed, Jul 29, 2015 at 08:43:47PM +0300, Marcel Apfelbaum wrote:
>> On 07/29/2015 08:20 PM, Michael S. Tsirkin wrote:
>>> On Wed, Jul 29, 2015 at 08:11:58PM +0300, Marcel Apfelbaum wrote:
>>>> Complete vhost-user negotiation by syncing the features
>>>> supported by the backend.
>>>>
>>>> Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
>>>> ---
>>>>   To be used on top of:
>>>>   [PATCH 0/4] vhost-user: protocol updates
>>>>   https://lists.gnu.org/archive/html/qemu-devel/2015-07/msg03842.html
>>>>
>>>>   Currently the vhost-user supported features are not evaluated.
>>>>   The way I see it, and please correct me, the best way to do
>>>>   this is to:
>>>>    1. get the backend features on vhost init
>>>>    2. Instead of simply copying them during features ack,
>>>>       check that that all backend features are supported by current QEMU
>>>>    3. All other code should remain the same.
>>>>
>>>>    Thanks,
>>>>    Marcel
>>>>
>>>>   hw/net/vhost_net.c     | 3 ++-
>>>>   hw/virtio/vhost-user.c | 4 ++--
>>>>   2 files changed, 4 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
>>>> index c864237..1ea5866 100644
>>>> --- a/hw/net/vhost_net.c
>>>> +++ b/hw/net/vhost_net.c
>>>> @@ -118,7 +118,8 @@ uint64_t vhost_net_get_features(struct vhost_net *net, uint64_t features)
>>>>
>>>>   void vhost_net_ack_features(struct vhost_net *net, uint64_t features)
>>>>   {
>>>> -    net->dev.acked_features = net->dev.backend_features;
>>>> +    vhost_ack_features(&net->dev, vhost_net_get_feature_bits(net),
>>>> +                       net->dev.backend_features);
>>>>       vhost_ack_features(&net->dev, vhost_net_get_feature_bits(net), features);
>>>
>>> So you ack it twice?
>> Not really, first call to vhost_ack_features acks the net->dev.backend_features,
>> the second one acks the guest features.
>>
>> The first call replaces the previous assignment that assumes that all backend features are
>> supported by QEMU.
>>
>> Thanks,
>> Marcel
>
> I think it's cleaner to whitelist backend features QEMU supports.
> I thought we did this, will look again.
I thought to do it the same as you started in vhost-init:

   if (__virtio_has_feature(msg.u64, VHOST_USER_F_PROTOCOL_FEATURES)) {
        dev->backend_features |= 1ULL << VHOST_USER_F_PROTOCOL_FEATURES;

However, we should check all possible flags, no? We have about 16 now.
And we'll need to manually add to white-list every new feature, seems error prone.
Anyway, I am open to suggestions.

Thanks,
Marcel


>>
>>>
>>>>   }
>>>>
>>>> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
>>>> index c4428a1..077457b 100644
>>>> --- a/hw/virtio/vhost-user.c
>>>> +++ b/hw/virtio/vhost-user.c
>>>> @@ -358,9 +358,9 @@ static int vhost_user_init(struct vhost_dev *dev, void *opaque)
>>>>           return err;
>>>>       }
>>>>
>>>> -    if (__virtio_has_feature(msg.u64, VHOST_USER_F_PROTOCOL_FEATURES)) {
>>>> -        dev->backend_features |= 1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
>>>> +    dev->backend_features = msg.u64;
>>>>
>>>> +    if (__virtio_has_feature(msg.u64, VHOST_USER_F_PROTOCOL_FEATURES)) {
>>>>           msg.request = VHOST_USER_GET_PROTOCOL_FEATURES;
>>>>           msg.flags = VHOST_USER_VERSION;
>>>>           msg.size = 0;
>>>> --
>>>> 2.1.0