[PATCH] hw/net/net_tx_pkt: Fix virtio header without checksum offloading

[Akihiko Odaki <akihiko.odaki@daynix.com>]

It is incorrect to have the VIRTIO_NET_HDR_F_NEEDS_CSUM set when
checksum offloading is disabled so clear the bit. Set the
VIRTIO_NET_HDR_F_DATA_VALID bit instead to tell the checksum is valid.

TCP/UDP checksum is usually offloaded when the peer requires virtio
headers because they can instruct the peer to compute checksum. However,
igb disables TX checksum offloading when a VF is enabled whether the
peer requires virtio headers because a transmitted packet can be routed
to it and it expects the packet has a proper checksum. Therefore, it
is necessary to have a correct virtio header even when checksum
offloading is disabled.

A real TCP/UDP checksum will be computed and saved in the buffer when
checksum offloading is disabled. The virtio specification requires to
set the packet checksum stored in the buffer to the TCP/UDP pseudo
header when the VIRTIO_NET_HDR_F_NEEDS_CSUM bit is set so the bit must
be cleared in that case.

The VIRTIO_NET_HDR_F_NEEDS_CSUM bit also tells to skip checksum
validation. Even if checksum offloading is disabled, it is desirable to
skip checksum validation because the checksum is always correct. Use the
VIRTIO_NET_HDR_F_DATA_VALID bit to claim the validity of the checksum.

Fixes: ffbd2dbd8e64 ("e1000e: Perform software segmentation for loopback")
Buglink: https://issues.redhat.com/browse/RHEL-23067
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
---
 hw/net/net_tx_pkt.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/net/net_tx_pkt.c b/hw/net/net_tx_pkt.c
index 2e5f58b3c9cc..c225cf706513 100644
--- a/hw/net/net_tx_pkt.c
+++ b/hw/net/net_tx_pkt.c
@@ -833,6 +833,9 @@ bool net_tx_pkt_send_custom(struct NetTxPkt *pkt, bool offload,
 
     if (offload || gso_type == VIRTIO_NET_HDR_GSO_NONE) {
         if (!offload && pkt->virt_hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
+            pkt->virt_hdr.flags =
+                (pkt->virt_hdr.flags & ~VIRTIO_NET_HDR_F_NEEDS_CSUM) |
+                VIRTIO_NET_HDR_F_DATA_VALID;
             net_tx_pkt_do_sw_csum(pkt, &pkt->vec[NET_TX_PKT_L2HDR_FRAG],
                                   pkt->payload_frags + NET_TX_PKT_PL_START_FRAG - 1,
                                   pkt->payload_len);

---
base-commit: ba49d760eb04630e7b15f423ebecf6c871b8f77b
change-id: 20240324-tx-c57d3c22ad73

Best regards,
-- 
Akihiko Odaki <akihiko.odaki@daynix.com>