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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 9 Jan 2025 02:40:55 -0500

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 <[email-masked]>
> 
> 
> 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?

> > ---
> >  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