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

[Akihiko Odaki]

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.od...@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