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Tsirkin" , netdev@vger.kernel.org, virtualization@lists.linux.dev, regressions@lists.linux.dev Subject: Re: [REGRESSION][BISECTED] virtio_net CSUM broken with Cloud Hypervisor In-Reply-To: References: <87y0ota32b.fsf@alyssa.is> Date: Thu, 30 Oct 2025 12:48:43 +0100 Message-ID: <87v7jwwqxw.fsf@alyssa.is> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha512; protocol="application/pgp-signature" --=-=-= Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Jason Wang writes: > On Thu, Oct 30, 2025 at 4:03=E2=80=AFAM Alyssa Ross wrote: >> >> Since 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support."), >> networking in Cloud Hypervisor is no longer working for me. >> >> I've narrowed down the problem to here: >> >> > @@ -2555,14 +2567,21 @@ static void virtnet_receive_done(struct virtne= t_info *vi, struct receive_queue * >> > if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report) >> > virtio_skb_set_hash(&hdr->hash_v1_hdr, skb); >> > >> > - if (flags & VIRTIO_NET_HDR_F_DATA_VALID) >> > - skb->ip_summed =3D CHECKSUM_UNNECESSARY; >> > + hdr->hdr.flags =3D flags; >> >> It looks like this was added because virtio_net_handle_csum_offload() >> looks at the flags from the hdr it's given, rather than having it passed >> separately, but it appears something later on relies on the previous >> value of hdr->hdr.flags. >> >> From my tracing, hdr->hdr.flags is set to either 0 or >> VIRTIO_NET_HDR_F_NEEDS_CSUM before this assignment, and flags is always >> 0, so in some cases VIRTIO_NET_HDR_F_NEEDS_CSUM now ends up being unset. > > Are you using XDP, if not there should be no change. > >> >> > + if (virtio_net_handle_csum_offload(skb, &hdr->hdr, vi->rx_tnl_cs= um)) { >> > + net_warn_ratelimited("%s: bad csum: flags: %x, gso_type:= %x rx_tnl_csum %d\n", >> > + dev->name, hdr->hdr.flags, >> > + hdr->hdr.gso_type, vi->rx_tnl_csum); >> > + goto frame_err; >> > + } >> >> If I change it to save the previous value of hdr->hdr.flags, and restore >> it again here, everything works again. >> >> Disabling offload_csum in Cloud Hypervisor is a usable workaround, >> because then hdr->hdr.flags is always 0 to begin with anyway. >> >> #regzbot introduced: 56a06bd40fab > > Is mergeable rx buffer enabled, if not, I wonder if this can be fixed by: > > https://marc.info/?l=3Dlinux-netdev&m=3D176170721926346&w=3D2 That patch does fix it, thank you! #regzbot fix: virtio-net: fix incorrect flags recording in big mode --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEARYKAB0WIQRV/neXydHjZma5XLJbRZGEIw/wogUCaQNQnAAKCRBbRZGEIw/w ohOQAQDLxZuMcKYN0k3GnM3RqtAZlaIoeA/1DoR+5ysX5/GVOwD9Hu8Nskv7B5H8 NgvMbITEXcfnNVlo/+iQCswKTUUMYwA= =WiqU -----END PGP SIGNATURE----- --=-=-=--