net: netkit: declare NETMEM_TX_NO_DMA mode

Some virtual devices like netkit (or ifb) never DMA and never touch frag
contents, they just forward the skb to another device. They are unable
to forward unreadable skbs, however, because they fail to pass TX
validation checks on dev->netmem_tx. The existing two-state
NETMEM_TX_NONE / NETMEM_TX_DMA doesn't give the TX validator enough
information to differentiate devices that will attempt DMA on the
unreadable skb from those that will simply route it untouched.

Add a third mode to the enum so drivers can indicate 1) if they have
netmem TX support, and 2) if they do, whether they are DMA-capable:

NETMEM_TX_NO_DMA - pass-through, device never DMAs

Widen dev->netmem_tx from a 1-bit field to 2 bits to fit the new value,
and declare netkit as NETMEM_TX_NO_DMA. Devmem TX support over these
devices comes in a follow-up patch.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260514-tcp-dm-netkit-v5-2-408c59b91e66@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index b7a4503f7cdb..bf3dd9b2c1a7 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1797,6 +1797,7 @@ enum netdev_stat_type {
 enum netmem_tx_mode {
 	NETMEM_TX_NONE,		/* no netmem TX support */
 	NETMEM_TX_DMA,		/* DMA-capable netmem TX (real HW) */
+	NETMEM_TX_NO_DMA,	/* no DMA, e.g. passthrough for virtual devs */
 };
 
 enum netdev_reg_state {
@@ -2143,7 +2144,7 @@ struct net_device {
 	struct_group(priv_flags_fast,
 		unsigned long		priv_flags:32;
 		unsigned long		lltx:1;
-		unsigned long		netmem_tx:1;
+		unsigned long		netmem_tx:2;
 	);
 	const struct net_device_ops *netdev_ops;
 	const struct header_ops *header_ops;