[PATCH] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM

[PATCH] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM

[Laurent Vivier]

According to the virtio-net specification, when the VIRTIO_NET_F_GUEST_CSUM
is negotiated, the device can set VIRTIO_NET_HDR_F_DATA_VALID in the
virtio-net header to indicate that packet checksums have been validated,
allowing the guest to skip verification. Without this feature, the device
must provide fully checksummed packets.

The vhost-user TCP and UDP paths were unconditionally skipping checksum
computation, regardless of whether GUEST_CSUM was negotiated. This
went undetected with Linux guests because Linux's virtio-net driver
honours VIRTIO_NET_HDR_F_DATA_VALID regardless of whether
VIRTIO_NET_F_GUEST_CSUM was negotiated, marking such packets as
CHECKSUM_UNNECESSARY and skipping verification.

iPXE, however, does not negotiate GUEST_CSUM, ignores the DATA_VALID
flag entirely, and always verifies checksums. This caused TCP
connections to fail: the SYN-ACK had a zero TCP checksum, iPXE rejected
it, and the connection timed out in SYN_RCVD.

Adding --pcap happened to mask the bug, because the pcap code path
forces checksum computation to ensure correct captures.

Offer VIRTIO_NET_F_GUEST_CSUM in the device features, and only skip
checksum computation when the guest has actually negotiated it. When
GUEST_CSUM is not negotiated, always compute valid checksums as required
by the specification.

We keep setting VIRTIO_NET_HDR_F_DATA_VALID unconditionally in
VU_HEADER: when GUEST_CSUM is negotiated, the flag lets the guest skip
checksum verification; when it is not, the spec says the guest should
ignore the flags field, so setting it is harmless.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---

Notes:
    Based-on: 20260323165259.1253482-1-lvivier@redhat.com

 tcp_vu.c     | 8 ++++++--
 udp_vu.c     | 6 ++++--
 vhost_user.c | 1 +
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/tcp_vu.c b/tcp_vu.c
index 776b47aea18c..2ab56f25a9be 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -172,7 +172,9 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
 		seq--;
 
 	payload = IOV_TAIL(flags_elem[0].in_sg, iov_cnt, VNET_HLEN);
-	tcp_fill_headers(c, conn, CONN_V4(conn), &payload, -1, seq, !*c->pcap);
+	tcp_fill_headers(c, conn, CONN_V4(conn), &payload, -1, seq,
+			 vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM) &&
+			 !*c->pcap);
 
 	if (*c->pcap)
 		pcap_iov(flags_elem[0].in_sg, iov_cnt, VNET_HLEN);
@@ -512,7 +514,9 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 			check = -1;
 		previous_dlen = dlen;
 
-		tcp_vu_prepare(c, conn, iov, iov_cnt, &check, !*c->pcap, push);
+		tcp_vu_prepare(c, conn, iov, iov_cnt, &check,
+			       vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM) &&
+			       !*c->pcap, push);
 
 		if (*c->pcap)
 			pcap_iov(iov, iov_cnt, VNET_HLEN);
diff --git a/udp_vu.c b/udp_vu.c
index 80391b4f8788..7ed271403481 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -223,10 +223,12 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
 			vu_set_vnethdr(iov_vu[0].iov_base, elem_used);
 			iov_drop_header(&data, VNET_HLEN);
 			udp_vu_prepare(c, &data, toside);
-			if (*c->pcap) {
+			if (!vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM) ||
+			    *c->pcap) {
 				udp_vu_csum(toside, &data);
-				pcap_iov(data.iov, data.cnt, data.off);
 			}
+			if (*c->pcap)
+				pcap_iov(data.iov, data.cnt, data.off);
 			vu_flush(vdev, vq, elem, elem_used);
 		}
 	}
diff --git a/vhost_user.c b/vhost_user.c
index 75665ec6522f..08a7b2d74099 100644
--- a/vhost_user.c
+++ b/vhost_user.c
@@ -322,6 +322,7 @@ static bool vu_get_features_exec(struct vu_dev *vdev,
 {
 	uint64_t features =
 		1ULL << VIRTIO_F_VERSION_1 |
+		1ULL << VIRTIO_NET_F_GUEST_CSUM |
 		1ULL << VIRTIO_NET_F_MRG_RXBUF |
 		1ULL << VHOST_F_LOG_ALL |
 		1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
-- 
2.53.0

Re: [PATCH] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM

[Laurent Vivier]

On 3/23/26 19:01, Laurent Vivier wrote:
> iPXE, however, does not negotiate GUEST_CSUM, ignores the DATA_VALID
> flag entirely, and always verifies checksums. This caused TCP
> connections to fail: the SYN-ACK had a zero TCP checksum, iPXE rejected
> it, and the connection timed out in SYN_RCVD.

And if you're wondering who's behind this laziness, check the author's name...

https://gitlab.com/qemu-project/ipxe/-/blob/master/src/drivers/net/virtio-net.c?ref_type=heads#L8

Thanks,
Laurent

Re: [PATCH] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM

[David Gibson]

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On Mon, Mar 23, 2026 at 07:01:49PM +0100, Laurent Vivier wrote:
> According to the virtio-net specification, when the VIRTIO_NET_F_GUEST_CSUM
> is negotiated, the device can set VIRTIO_NET_HDR_F_DATA_VALID in the
> virtio-net header to indicate that packet checksums have been validated,
> allowing the guest to skip verification. Without this feature, the device
> must provide fully checksummed packets.
> 
> The vhost-user TCP and UDP paths were unconditionally skipping checksum
> computation, regardless of whether GUEST_CSUM was negotiated. This
> went undetected with Linux guests because Linux's virtio-net driver
> honours VIRTIO_NET_HDR_F_DATA_VALID regardless of whether
> VIRTIO_NET_F_GUEST_CSUM was negotiated, marking such packets as
> CHECKSUM_UNNECESSARY and skipping verification.
> 
> iPXE, however, does not negotiate GUEST_CSUM, ignores the DATA_VALID
> flag entirely, and always verifies checksums. This caused TCP
> connections to fail: the SYN-ACK had a zero TCP checksum, iPXE rejected
> it, and the connection timed out in SYN_RCVD.
> 
> Adding --pcap happened to mask the bug, because the pcap code path
> forces checksum computation to ensure correct captures.
> 
> Offer VIRTIO_NET_F_GUEST_CSUM in the device features, and only skip
> checksum computation when the guest has actually negotiated it. When
> GUEST_CSUM is not negotiated, always compute valid checksums as required
> by the specification.
> 
> We keep setting VIRTIO_NET_HDR_F_DATA_VALID unconditionally in
> VU_HEADER: when GUEST_CSUM is negotiated, the flag lets the guest skip
> checksum verification; when it is not, the spec says the guest should
> ignore the flags field, so setting it is harmless.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
> 
> Notes:
>     Based-on: 20260323165259.1253482-1-lvivier@redhat.com
> 
>  tcp_vu.c     | 8 ++++++--
>  udp_vu.c     | 6 ++++--
>  vhost_user.c | 1 +
>  3 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/tcp_vu.c b/tcp_vu.c
> index 776b47aea18c..2ab56f25a9be 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -172,7 +172,9 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  		seq--;
>  
>  	payload = IOV_TAIL(flags_elem[0].in_sg, iov_cnt, VNET_HLEN);
> -	tcp_fill_headers(c, conn, CONN_V4(conn), &payload, -1, seq, !*c->pcap);
> +	tcp_fill_headers(c, conn, CONN_V4(conn), &payload, -1, seq,
> +			 vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM) &&
> +			 !*c->pcap);
>  
>  	if (*c->pcap)
>  		pcap_iov(flags_elem[0].in_sg, iov_cnt, VNET_HLEN);
> @@ -512,7 +514,9 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  			check = -1;
>  		previous_dlen = dlen;
>  
> -		tcp_vu_prepare(c, conn, iov, iov_cnt, &check, !*c->pcap, push);
> +		tcp_vu_prepare(c, conn, iov, iov_cnt, &check,
> +			       vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM) &&
> +			       !*c->pcap, push);
>  
>  		if (*c->pcap)
>  			pcap_iov(iov, iov_cnt, VNET_HLEN);
> diff --git a/udp_vu.c b/udp_vu.c
> index 80391b4f8788..7ed271403481 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -223,10 +223,12 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
>  			vu_set_vnethdr(iov_vu[0].iov_base, elem_used);
>  			iov_drop_header(&data, VNET_HLEN);
>  			udp_vu_prepare(c, &data, toside);
> -			if (*c->pcap) {
> +			if (!vu_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM) ||
> +			    *c->pcap) {
>  				udp_vu_csum(toside, &data);
> -				pcap_iov(data.iov, data.cnt, data.off);
>  			}
> +			if (*c->pcap)
> +				pcap_iov(data.iov, data.cnt, data.off);
>  			vu_flush(vdev, vq, elem, elem_used);
>  		}
>  	}
> diff --git a/vhost_user.c b/vhost_user.c
> index 75665ec6522f..08a7b2d74099 100644
> --- a/vhost_user.c
> +++ b/vhost_user.c
> @@ -322,6 +322,7 @@ static bool vu_get_features_exec(struct vu_dev *vdev,
>  {
>  	uint64_t features =
>  		1ULL << VIRTIO_F_VERSION_1 |
> +		1ULL << VIRTIO_NET_F_GUEST_CSUM |
>  		1ULL << VIRTIO_NET_F_MRG_RXBUF |
>  		1ULL << VHOST_F_LOG_ALL |
>  		1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
> -- 
> 2.53.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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Re: [PATCH] vhost_user: Offer VIRTIO_NET_F_GUEST_CSUM

[David Gibson]

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On Mon, Mar 23, 2026 at 07:15:36PM +0100, Laurent Vivier wrote:
> On 3/23/26 19:01, Laurent Vivier wrote:
> > iPXE, however, does not negotiate GUEST_CSUM, ignores the DATA_VALID
> > flag entirely, and always verifies checksums. This caused TCP
> > connections to fail: the SYN-ACK had a zero TCP checksum, iPXE rejected
> > it, and the connection timed out in SYN_RCVD.
> 
> And if you're wondering who's behind this laziness, check the author's name...
> 
> https://gitlab.com/qemu-project/ipxe/-/blob/master/src/drivers/net/virtio-net.c?ref_type=heads#L8

Aw man, not that guy!

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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