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net: netkit: declare NETMEM_TX_NO_DMA mode

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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>
This commit is contained in:
Bobby Eshleman
2026-05-14 10:22:29 -07:00
committed by Jakub Kicinski
parent 7d3ab852dc
commit 6ce2bb0480
5 changed files with 10 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
Documentation/networking/net_cachelines/net_device.rst
View File

@@ -10,7 +10,7 @@ Type Name fastpath_tx_acce
=================================== =========================== =================== =================== ===================================================================================
unsigned_long:32 priv_flags read_mostly __dev_queue_xmit(tx)
unsigned_long:1 lltx read_mostly HARD_TX_LOCK,HARD_TX_TRYLOCK,HARD_TX_UNLOCK(tx)
unsigned long:1 netmem_tx:1; read_mostly
unsigned_long:2 netmem_tx:2; read_mostly
char name[16]
struct netdev_name_node* name_node
struct dev_ifalias* ifalias

3
Documentation/networking/netmem.rst
View File

@@ -99,3 +99,6 @@ Driver TX Requirements
appropriate mode:
- `NETMEM_TX_DMA`: for physical devices that perform DMA.
- `NETMEM_TX_NO_DMA`: for virtual or passthrough devices that do
not DMA, but still support handling of netmem-backed skbs.

3
Documentation/translations/zh_CN/networking/netmem.rst
View File

@@ -92,3 +92,6 @@ dma-mapping API 去处理。
2. 驱动程序应将 `netdev->netmem_tx` 设置为适当的模式：
- `NETMEM_TX_DMA`：适用于执行 DMA 的物理设备。
- `NETMEM_TX_NO_DMA`：适用于不执行 DMA 的虚拟或透传设备，但仍支持
处理 netmem 支持的 skb。

1
drivers/net/netkit.c
View File

@@ -466,6 +466,7 @@ static void netkit_setup(struct net_device *dev)
dev->priv_flags |= IFF_NO_QUEUE;
dev->priv_flags |= IFF_DISABLE_NETPOLL;
dev->lltx = true;
dev->netmem_tx = NETMEM_TX_NO_DMA;
dev->netdev_ops = &netkit_netdev_ops;
dev->ethtool_ops = &netkit_ethtool_ops;

3
include/linux/netdevice.h
View File

@@ -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;