Re: [PATCH v2 3/3] tun: Set num_buffers for virtio 1.0

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2025/01/09 16:40, Michael S. Tsirkin wrote:
> On Thu, Jan 09, 2025 at 02:32:25AM -0500, Michael S. Tsirkin wrote:
>> On Thu, Jan 09, 2025 at 03:58:45PM +0900, Akihiko Odaki wrote:
>>> The specification says the device MUST set num_buffers to 1 if
>>> VIRTIO_NET_F_MRG_RXBUF has not been negotiated.
>>>
>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>>
>>
>> How do we know this is v1 and not v0? Confused.
> 
> Ah I got it, you assume userspace will over-write it
> if VIRTIO_NET_F_MRG_RXBUF is set.
> If we are leaving this up to userspace, why not let it do
> it always?

tun may be used with vhost_net, which does not set the field.

> 
>>> ---
>>>   drivers/net/tap.c      |  2 +-
>>>   drivers/net/tun.c      |  6 ++++--
>>>   drivers/net/tun_vnet.c | 14 +++++++++-----
>>>   drivers/net/tun_vnet.h |  4 ++--
>>>   4 files changed, 16 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>>> index 60804855510b..fe9554ee5b8b 100644
>>> --- a/drivers/net/tap.c
>>> +++ b/drivers/net/tap.c
>>> @@ -713,7 +713,7 @@ static ssize_t tap_put_user(struct tap_queue *q,
>>>   	int total;
>>>   
>>>   	if (q->flags & IFF_VNET_HDR) {
>>> -		struct virtio_net_hdr vnet_hdr;
>>> +		struct virtio_net_hdr_v1 vnet_hdr;
>>>   
>>>   		vnet_hdr_len = READ_ONCE(q->vnet_hdr_sz);
>>>   
>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>> index dbf0dee92e93..f211d0580887 100644
>>> --- a/drivers/net/tun.c
>>> +++ b/drivers/net/tun.c
>>> @@ -1991,7 +1991,9 @@ static ssize_t tun_put_user_xdp(struct tun_struct *tun,
>>>   	size_t total;
>>>   
>>>   	if (tun->flags & IFF_VNET_HDR) {
>>> -		struct virtio_net_hdr gso = { 0 };
>>> +		struct virtio_net_hdr_v1 gso = {
>>> +			.num_buffers = __virtio16_to_cpu(true, 1)
>>> +		};
>>>   
>>>   		vnet_hdr_sz = READ_ONCE(tun->vnet_hdr_sz);
>>>   		ret = tun_vnet_hdr_put(vnet_hdr_sz, iter, &gso);
>>> @@ -2044,7 +2046,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>>>   	}
>>>   
>>>   	if (vnet_hdr_sz) {
>>> -		struct virtio_net_hdr gso;
>>> +		struct virtio_net_hdr_v1 gso;
>>>   
>>>   		ret = tun_vnet_hdr_from_skb(tun->flags, tun->dev, skb, &gso);
>>>   		if (ret < 0)
>>> diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c
>>> index ffb2186facd3..a7a7989fae56 100644
>>> --- a/drivers/net/tun_vnet.c
>>> +++ b/drivers/net/tun_vnet.c
>>> @@ -130,15 +130,17 @@ int tun_vnet_hdr_get(int sz, unsigned int flags, struct iov_iter *from,
>>>   EXPORT_SYMBOL_GPL(tun_vnet_hdr_get);
>>>   
>>>   int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
>>> -		     const struct virtio_net_hdr *hdr)
>>> +		     const struct virtio_net_hdr_v1 *hdr)
>>>   {
>>> +	int content_sz = MIN(sizeof(*hdr), sz);
>>> +
>>>   	if (iov_iter_count(iter) < sz)
>>>   		return -EINVAL;
>>>   
>>> -	if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr))
>>> +	if (copy_to_iter(hdr, content_sz, iter) != content_sz)
>>>   		return -EFAULT;
>>>   
>>> -	if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr))
>>> +	if (iov_iter_zero(sz - content_sz, iter) != sz - content_sz)
>>>   		return -EFAULT;
>>>   
>>>   	return 0;
>>> @@ -154,11 +156,11 @@ EXPORT_SYMBOL_GPL(tun_vnet_hdr_to_skb);
>>>   
>>>   int tun_vnet_hdr_from_skb(unsigned int flags, const struct net_device *dev,
>>>   			  const struct sk_buff *skb,
>>> -			  struct virtio_net_hdr *hdr)
>>> +			  struct virtio_net_hdr_v1 *hdr)
>>>   {
>>>   	int vlan_hlen = skb_vlan_tag_present(skb) ? VLAN_HLEN : 0;
>>>   
>>> -	if (virtio_net_hdr_from_skb(skb, hdr,
>>> +	if (virtio_net_hdr_from_skb(skb, (struct virtio_net_hdr *)hdr,
>>>   				    tun_vnet_is_little_endian(flags), true,
>>>   				    vlan_hlen)) {
>>>   		struct skb_shared_info *sinfo = skb_shinfo(skb);
>>> @@ -176,6 +178,8 @@ int tun_vnet_hdr_from_skb(unsigned int flags, const struct net_device *dev,
>>>   		return -EINVAL;
>>>   	}
>>>   
>>> +	hdr->num_buffers = 1;
>>> +
>>>   	return 0;
>>>   }
>>>   EXPORT_SYMBOL_GPL(tun_vnet_hdr_from_skb);
>>> diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
>>> index 2dfdbe92bb24..d8fd94094227 100644
>>> --- a/drivers/net/tun_vnet.h
>>> +++ b/drivers/net/tun_vnet.h
>>> @@ -12,13 +12,13 @@ int tun_vnet_hdr_get(int sz, unsigned int flags, struct iov_iter *from,
>>>   		     struct virtio_net_hdr *hdr);
>>>   
>>>   int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
>>> -		     const struct virtio_net_hdr *hdr);
>>> +		     const struct virtio_net_hdr_v1 *hdr);
>>>   
>>>   int tun_vnet_hdr_to_skb(unsigned int flags, struct sk_buff *skb,
>>>   			const struct virtio_net_hdr *hdr);
>>>   
>>>   int tun_vnet_hdr_from_skb(unsigned int flags, const struct net_device *dev,
>>>   			  const struct sk_buff *skb,
>>> -			  struct virtio_net_hdr *hdr);
>>> +			  struct virtio_net_hdr_v1 *hdr);
>>>   
>>>   #endif /* TUN_VNET_H */
>>>
>>> -- 
>>> 2.47.1
>