Re: [PATCH net v1] virtio_net: Introduce skb_vnet_common_hdr to avoid typecasting

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On 2023-08-15 p.m.2:13, Willem de Bruijn wrote:
External email: Use caution opening links or attachments

On Tue, Aug 15, 2023 at 12:29 PM Simon Horman <horms@xxxxxxxxxx> wrote:

On Tue, Aug 15, 2023 at 11:09:02AM -0400, Feng Liu wrote:

To clarify: In general new Networking features go via the net-next tree,
while bug fixes go via the net tree. I was suggesting this
is more appropriate for net-next, and that should be reflected in the

         Subject: [PATCH net-next] ...

Sorry for not being clearer the first time around.

Right, this should go to net-next.

Will do, thanks

diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index 12c1c9699935..db40f93ae8b3 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -201,6 +201,13 @@ struct virtio_net_hdr_mrg_rxbuf {
        struct virtio_net_hdr hdr;
        __virtio16 num_buffers; /* Number of merged rx buffers */
+struct virtio_net_common_hdr {
+     union {
+             struct virtio_net_hdr_mrg_rxbuf mrg_hdr;
+             struct virtio_net_hdr_v1_hash hash_v1_hdr;
+     };

Does this belong in the UAPI?
I would have assumed it's a Kernel implementation detail.

The existing codes, virtio_net.h is in uapi/linux/, I added the new
structure and followed existing code. My modification is related to Kernel
implementation detail now.

The header you have modified forms part of the userspace API (UAPI).
Perhaps there is something about virtio_net that makes this correct, but it
seems to me that kernel-internal details don't belong there.

FWIW, I ran into similar issues before in a draft that added timestamp
support [1]

If we're going to change this structure, we should do it in a way that
is forward proof to future extensions to the virtio spec and with that
the fields in this struct. Especially in UAPI.

Is virtio_net_hdr_v1_hash the latest virtio-spec compliant header? And
do we expect for v1.3 to just add some fields to this?

The struct comment of virtio_net_hdr_v1 states "This is
bitwise-equivalent to the legacy struct virtio_net_hdr_mrg_rxbuf, only
flattened.". I don't quite understand what the flattening bought, vs
having struct virtio_net_hdr as first member. Another difference may
be the endianness between legacy (0.9) and v1.0+.

Since legacy virtio will no longer be modified, I don't think there is
much value is exposing this new union as UAPI. I do appreciate the
benefit to the implementation.

Hi, William and Simon

Thanks for the detailed explanation.

I kept virtio_net_hdr_mrg_rxbuf and virtio_net_hdr_v1_hash structures in virtio_net.h, which can be forward compatible with existing user applications which use these structures.

After checking kernel code, the virtio_net_hdr_v1_hash structure does only add new members to virtio_net_hdr_mrg_rxbuf, so the spec should only add new members, otherwise there will be compatibility problems in struct virtio_net_hdr_v1_hash structure.

struct virtio_net_hdr_v1_hash {
	struct virtio_net_hdr_v1 hdr; /*same size as virtio_net_hdr*/
	__le32 hash_value; /*new member*/
	__le16 hash_report; /*new member*/
	__le16 padding;	/*new member*/

virtio_net_hdr_v1_hash cannot use virtio_net_hdr as the first member, because in virtio_net_hdr_v1, csum_start and csum_offset are stored in union as a structure, and virtio_net_hdr cannot be used instead.

struct virtio_net_hdr_v1 {
	union {
		struct {
			__virtio16 csum_start;
			__virtio16 csum_offset;
	__virtio16 num_buffers;	/* Number of merged rx buffers */

struct virtio_net_hdr {
	__virtio16 csum_start;	
	__virtio16 csum_offset;	

In addition, I put this new structure virtio_net_common_hdr in uapi, hoping it could be used in future user space application to avoid potential risks caused by type coercion (such as the problems mentioned in the patch description ). So I think it should be in this header file.
What do you think?

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