[PATCH net] gve: DQO: accept TSO packets with non-protocol gso_type bits

[Hannu Varjoranta <hannu@varjosoft.com>]

Since commit 014c607f86ab ("gve: add support for UDP GSO for DQO
format"), gve_prep_tso() matches shinfo->gso_type exactly. gso_type is
a bitmask, though: SKB_GSO_DODGY is set on every GSO packet that comes
from tun/tap, packet sockets or other untrusted sources, and
SKB_GSO_TCP_ECN and SKB_GSO_TCP_FIXEDID can be set as well. Such packets
hit the default case, gve_tx_add_skb_dqo() fails and gve_try_tx_skb()
drops them, counting them in tx_dropped.

These packets do reach the driver: tcp_gso_segment() passes DODGY skbs
through unsegmented to devices that support TSO, after recomputing
gso_segs, so drivers must tolerate the bit.

This breaks virtual machines behind a tap on GCE instances using the DQO
queue formats. Every TSO packet forwarded from a guest (gso_type
SKB_GSO_TCPV4 | SKB_GSO_DODGY) is dropped, and guest uploads slow to a
crawl of retransmissions or stall entirely, while the host's own TSO
traffic (gso_type SKB_GSO_TCPV4) is unaffected. On an n4 instance
(DQO-QPL) with a Cloud Hypervisor guest, a 64 MB upload from the guest
went from a 60 s timeout at ~0.9 MB/s to 0.19 s with this change, with
tx_dropped no longer increasing.

Restore the bitmask test that commit 1b9f75634441 ("gve: ignore
nonrelevant GSO type bits when processing TSO headers") introduced for
the same problem, keeping the UDP GSO support.

Fixes: 014c607f86ab ("gve: add support for UDP GSO for DQO format")
Signed-off-by: Hannu Varjoranta <hannu@varjosoft.com>
---
 drivers/net/ethernet/google/gve/gve_tx_dqo.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/google/gve/gve_tx_dqo.c b/drivers/net/ethernet/google/gve/gve_tx_dqo.c
index 80ab0a449ff5..675c7644817a 100644
--- a/drivers/net/ethernet/google/gve/gve_tx_dqo.c
+++ b/drivers/net/ethernet/google/gve/gve_tx_dqo.c
@@ -596,21 +596,20 @@ static int gve_prep_tso(struct sk_buff *skb)
 	l4_start = skb_transport_offset(skb);
 	paylen = skb->len - l4_start;
 
-	switch (shinfo->gso_type) {
-	case SKB_GSO_TCPV4:
-	case SKB_GSO_TCPV6:
+	/* gso_type is a bitmask: SKB_GSO_DODGY, SKB_GSO_TCP_ECN and
+	 * SKB_GSO_TCP_FIXEDID may be set alongside the protocol bit.
+	 */
+	if (shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)) {
 		tcp = tcp_hdr(skb);
 		csum_replace_by_diff(&tcp->check,
 				     (__force __wsum)htonl(paylen));
 		header_len = skb_tcp_all_headers(skb);
-		break;
-	case SKB_GSO_UDP_L4:
+	} else if (shinfo->gso_type & SKB_GSO_UDP_L4) {
 		udp = udp_hdr(skb);
 		csum_replace_by_diff(&udp->check,
 				     (__force __wsum)htonl(paylen));
 		header_len = sizeof(struct udphdr) + l4_start;
-		break;
-	default:
+	} else {
 		return -EINVAL;
 	}
 
-- 
2.55.0