> -----Original Message-----
> From: Michael S. Tsirkin <[email protected]>
> Sent: Sunday, February 1, 2026 10:18 AM
> To: Chia-Yu Chang (Nokia) <[email protected]>
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> [email protected]; [email protected]; Koen De Schepper (Nokia)
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> Subject: Re: [PATCH v1 net-next 3/3] virtio_net: Accurate ECN flag in
> virtio_net_hdr
>
>
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>
> Thanks for the patch! Yet something to improve:
>
> On Sat, Jan 31, 2026 at 11:55:10PM +0100, [email protected]
> wrote:
> > From: Chia-Yu Chang <[email protected]>
> >
> > Unlike RFC 3168 ECN, accurate ECN uses the CWR flag as part of the ACE
> > field to count new packets with CE mark; however, it will be corrupted
> > by the RFC 3168 ECN-aware TSO. Therefore, fallback shall be applied by
> > seting NETIF_F_GSO_ACCECN to ensure that the CWR flag should not be
> > changed within a super-skb.
> >
> > To apply the aforementieond new AccECN GSO for virtio, new featue bits
> > for host and guest are added for feature negotiation between driver
> > and device. And the translation of Accurate ECN GSO flag between
> > virtio_net_hdr and skb header for NETIF_F_GSO_ACCECN is also added to
> > avoid CWR flag corruption due to RFC3168 ECN TSO.
> >
> > Signed-off-by: Chia-Yu Chang <[email protected]>
>
>
> To the best of my understanding, this is a new feature - support for
> VIRTIO_NET_F_HOST_ACCECN, VIRTIO_NET_F_GUEST_ACCECN?
> The commit log makes it sound like it fixes some behaviour for existing
> hardware, but that is not the case.
>
Thansk for the feedback, I will update commit message in v3.
>
> > ---
> > v2:
> > - Replace VIRTIO_NET_HDR_GSO_ECN with VIRTIO_NET_HDR_GSO_ECN_FLAGS
>
> but where is v2? this is v1...
I shall update this version as v2, will do at the next version.
[...]
> > diff --git a/include/uapi/linux/virtio_net.h
> > b/include/uapi/linux/virtio_net.h index 1db45b01532b..af5bfe45aa1f
> > 100644
> > --- a/include/uapi/linux/virtio_net.h
> > +++ b/include/uapi/linux/virtio_net.h
> > @@ -56,6 +56,8 @@
> > #define VIRTIO_NET_F_MQ 22 /* Device supports Receive Flow
> > * Steering */
> > #define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */
> > +#define VIRTIO_NET_F_HOST_ACCECN 25 /* Host can handle GSO of AccECN
> > +*/ #define VIRTIO_NET_F_GUEST_ACCECN 26 /* Guest can handle GSO of
> > +AccECN */
> > #define VIRTIO_NET_F_DEVICE_STATS 50 /* Device can provide
> > device-level statistics. */ #define VIRTIO_NET_F_VQ_NOTF_COAL 52 /* Device
> > supports virtqueue notification coalescing */
> > #define VIRTIO_NET_F_NOTF_COAL 53 /* Device supports
> > notifications coalescing */
> > @@ -165,6 +167,9 @@ struct virtio_net_hdr_v1 { #define
> > VIRTIO_NET_HDR_GSO_UDP_TUNNEL (VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 | \
> > VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6)
> > #define VIRTIO_NET_HDR_GSO_ECN 0x80 /* TCP has ECN set */
> > +#define VIRTIO_NET_HDR_GSO_ACCECN 0x10 /* TCP AccECN segmentation */
> > +#define VIRTIO_NET_HDR_GSO_ECN_FLAGS (VIRTIO_NET_HDR_GSO_ECN | \
> > + VIRTIO_NET_HDR_GSO_ACCECN)
> > __u8 gso_type;
> > __virtio16 hdr_len; /* Ethernet + IP + tcp/udp hdrs */
> > __virtio16 gso_size; /* Bytes to append to hdr_len per frame */
>
>
> UAPI changes need to be added to the virtio spec.
> Pls get this approved by the virtio TC.
> Thanks!
There were some discussions last October in [email protected]
mailing list.
At that moment, it is suggested to make Linux kernel accept new comments for
SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN first.
So, could virtio-spec colleague give your feedback? (Parav?).
Otherwise, the CWR handling of virtio will be wrong after all Accurate ECN
commits are merged in Linux.
Chia-Yu
