netdev - Re: [PATCH net-next 3/8] vhost-net: allow configuring extended features

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Message-ID: <546a1ee3-7003-4acf-879f-d67f65b534c2@redhat.com>
Date: Thu, 29 May 2025 13:10:42 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: netdev@...r.kernel.org, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, "Michael S. Tsirkin" <mst@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>
Subject: Re: [PATCH net-next 3/8] vhost-net: allow configuring extended
 features

On 5/27/25 5:56 AM, Jason Wang wrote:
> On Mon, May 26, 2025 at 6:57 PM Paolo Abeni <pabeni@...hat.com> wrote:
>> On 5/26/25 2:47 AM, Jason Wang wrote:
>>> On Wed, May 21, 2025 at 6:33 PM Paolo Abeni <pabeni@...hat.com> wrote:
>>>>
>>>> Use the extended feature type for 'acked_features' and implement
>>>> two new ioctls operation to get and set the extended features.
>>>>
>>>> Note that the legacy ioctls implicitly truncate the negotiated
>>>> features to the lower 64 bits range.
>>>>
>>>> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
>>>> ---
>>>>  drivers/vhost/net.c        | 26 +++++++++++++++++++++++++-
>>>>  drivers/vhost/vhost.h      |  2 +-
>>>>  include/uapi/linux/vhost.h |  8 ++++++++
>>>>  3 files changed, 34 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>>> index 7cbfc7d718b3f..b894685dded3e 100644
>>>> --- a/drivers/vhost/net.c
>>>> +++ b/drivers/vhost/net.c
>>>> @@ -77,6 +77,10 @@ enum {
>>>>                          (1ULL << VIRTIO_F_RING_RESET)
>>>>  };
>>>>
>>>> +#ifdef VIRTIO_HAS_EXTENDED_FEATURES
>>>> +#define VHOST_NET_FEATURES_EX VHOST_NET_FEATURES
>>>> +#endif
>>>> +
>>>>  enum {
>>>>         VHOST_NET_BACKEND_FEATURES = (1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2)
>>>>  };
>>>> @@ -1614,7 +1618,7 @@ static long vhost_net_reset_owner(struct vhost_net *n)
>>>>         return err;
>>>>  }
>>>>
>>>> -static int vhost_net_set_features(struct vhost_net *n, u64 features)
>>>> +static int vhost_net_set_features(struct vhost_net *n, virtio_features_t features)
>>>>  {
>>>>         size_t vhost_hlen, sock_hlen, hdr_len;
>>>>         int i;
>>>> @@ -1704,6 +1708,26 @@ static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
>>>>                 if (features & ~VHOST_NET_FEATURES)
>>>>                         return -EOPNOTSUPP;
>>>>                 return vhost_net_set_features(n, features);
>>>> +#ifdef VIRTIO_HAS_EXTENDED_FEATURES
>>>
>>> Vhost doesn't depend on virtio. But this invents a dependency, and I
>>> don't understand why we need to do that.
>>
>> What do you mean with "dependency" here? vhost has already a build
>> dependency vs virtio, including several virtio headers. It has also a
>> logical dependency, using several virtio features.
>>
>> Do you mean a build dependency? this change does not introduce such a thing.
> 
> I mean vhost can be built without virtio drivers. So old vhost can run
> new virtio drivers on top. So I don't see why vhost needs to check if
> virtio of the same source tree supports 128 bit or not.
> 
> We can just accept an array of features now as
> 
> 1) the changes are limited to vhost so it wouldn't be too much
> 2) we don't have to have VHOST_GET_FEATURES_EX2 in the future.

AFAICS the ioctl() interface code wise only impacts on the device
implementing extended features support, I guess it could be changed to
to something alike:

struct vhost_virtio_features {
	__u64 count;
	__u64 features[];
};

#define VHOST_GET_FEATURES_VECTOR _IOR(VHOST_VIRTIO, 0x83, struct
vhost_virtio_features)
#define VHOST_SET_FEATURES_VECTOR _IOW(VHOST_VIRTIO, 0x83, struct
vhost_virtio_features)

I could drop the above #ifdef, and the implementation would copy in/out
only the known/supported number of features.

WDYT?

/P