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Tue, 25 Nov 2025 09:59:04 -0800 (PST) Received: from [192.168.88.32] ([212.105.153.231]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42cb7f363b0sm36115430f8f.13.2025.11.25.09.59.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 25 Nov 2025 09:59:04 -0800 (PST) Message-ID: <8492bf2b-0fbc-4819-ab7a-dfe860cca412@redhat.com> Date: Tue, 25 Nov 2025 18:59:03 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next] virtio-net: avoid unnecessary checksum calculation on guest RX From: Paolo Abeni To: Jon Kohler , "Michael S. Tsirkin" , Jason Wang , Xuan Zhuo , =?UTF-8?Q?Eugenio_P=C3=A9rez?= , virtualization@lists.linux.dev, linux-kernel@vger.kernel.org Cc: "netdev@vger.kernel.org" References: <20251125175117.995179-1-jon@nutanix.com> <276828c5-72cb-4f5c-bc6f-7937aa6b6303@redhat.com> Content-Language: en-US In-Reply-To: <276828c5-72cb-4f5c-bc6f-7937aa6b6303@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/25/25 6:57 PM, Paolo Abeni wrote: > On 11/25/25 6:51 PM, Jon Kohler wrote: >> Commit a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP >> GSO tunneling.") inadvertently altered checksum offload behavior >> for guests not using UDP GSO tunneling. >> >> Before, tun_put_user called tun_vnet_hdr_from_skb, which passed >> has_data_valid = true to virtio_net_hdr_from_skb. >> >> After, tun_put_user began calling tun_vnet_hdr_tnl_from_skb instead, >> which passes has_data_valid = false into both call sites. >> >> This caused virtio hdr flags to not include VIRTIO_NET_HDR_F_DATA_VALID >> for SKBs where skb->ip_summed == CHECKSUM_UNNECESSARY. As a result, >> guests are forced to recalculate checksums unnecessarily. >> >> Restore the previous behavior by ensuring has_data_valid = true is >> passed in the !tnl_gso_type case. >> >> Cc: Paolo Abeni >> Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.") >> Signed-off-by: Jon Kohler >> --- >> include/linux/virtio_net.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h >> index b673c31569f3..570c6dd1666d 100644 >> --- a/include/linux/virtio_net.h >> +++ b/include/linux/virtio_net.h >> @@ -394,7 +394,7 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb, >> tnl_gso_type = skb_shinfo(skb)->gso_type & (SKB_GSO_UDP_TUNNEL | >> SKB_GSO_UDP_TUNNEL_CSUM); >> if (!tnl_gso_type) >> - return virtio_net_hdr_from_skb(skb, hdr, little_endian, false, >> + return virtio_net_hdr_from_skb(skb, hdr, little_endian, true, >> vlan_hlen); >> >> /* Tunnel support not negotiated but skb ask for it. */ > > virtio_net_hdr_tnl_from_skb() is used also by the virtio_net driver, > which in turn must not use VIRTIO_NET_HDR_F_DATA_VALID on tx. > > I think you need to add another argument to > virtio_net_hdr_tnl_from_skb(), or possibly implement a separate helper > to take care of csum offload - the symmetric of > virtio_net_handle_csum_offload(). > > Also you need to CC netdev, otherwise the patch will not be processed by > patchwork. whoops... I almost forgot... this is 'net' material, the subj prefix should be adjusted accordingly. Thanks, Paolo