Subject: Re: [PATCH net-next v4 6/9] net: enable driver support for netmem TX

From: Stanislav Fomichev <[email-masked]>
Date: Thu, 20 Feb 2025 11:13:48 -0800

On 02/20, Mina Almasry wrote:
> Drivers need to make sure not to pass netmem dma-addrs to the
> dma-mapping API in order to support netmem TX.
> 
> Add helpers and netmem_dma_*() helpers that enables special handling of
> netmem dma-addrs that drivers can use.
> 
> Document in netmem.rst what drivers need to do to support netmem TX.
> 
> Signed-off-by: Mina Almasry <[email-masked]>
> 
> ---
> 
> v4:
> - New patch
> ---
>  .../networking/net_cachelines/net_device.rst  |  1 +
>  Documentation/networking/netdev-features.rst  |  5 +++++
>  Documentation/networking/netmem.rst           | 14 +++++++++++--
>  include/linux/netdevice.h                     |  2 ++
>  include/net/netmem.h                          | 20 +++++++++++++++++++
>  5 files changed, 40 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/networking/net_cachelines/net_device.rst b/Documentation/networking/net_cachelines/net_device.rst
> index 15e31ece675f..e3043b033647 100644
> --- a/Documentation/networking/net_cachelines/net_device.rst
> +++ b/Documentation/networking/net_cachelines/net_device.rst
> @@ -10,6 +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
>  char                                name[16]
>  struct netdev_name_node*            name_node
>  struct dev_ifalias*                 ifalias
> diff --git a/Documentation/networking/netdev-features.rst b/Documentation/networking/netdev-features.rst
> index 5014f7cc1398..02bd7536fc0c 100644
> --- a/Documentation/networking/netdev-features.rst
> +++ b/Documentation/networking/netdev-features.rst
> @@ -188,3 +188,8 @@ Redundancy) frames from one port to another in hardware.
>  This should be set for devices which duplicate outgoing HSR (High-availability
>  Seamless Redundancy) or PRP (Parallel Redundancy Protocol) tags automatically
>  frames in hardware.
> +
> +* netmem-tx
> +
> +This should be set for devices which support netmem TX. See
> +Documentation/networking/netmem.rst
> diff --git a/Documentation/networking/netmem.rst b/Documentation/networking/netmem.rst
> index 7de21ddb5412..43054d44c407 100644
> --- a/Documentation/networking/netmem.rst
> +++ b/Documentation/networking/netmem.rst
> @@ -19,8 +19,8 @@ Benefits of Netmem :
>  * Simplified Development: Drivers interact with a consistent API,
>    regardless of the underlying memory implementation.
>  
> -Driver Requirements
> -===================
> +Driver RX Requirements
> +======================
>  
>  1. The driver must support page_pool.
>  
> @@ -77,3 +77,13 @@ Driver Requirements
>     that purpose, but be mindful that some netmem types might have longer
>     circulation times, such as when userspace holds a reference in zerocopy
>     scenarios.
> +
> +Driver TX Requirements
> +======================
> +
> +1. Driver should use netmem_dma_unmap_page_attrs() in lieu of
> +   dma_unmap_page[_attrs](), and netmem_dma_unmap_addr_set() in lieu of
> +   dma_unmap_addr_set(). The netmem variants will handle netmems that should
> +   not be dma-unmapped by the driver, such as dma-buf netmems.

Not all drivers use dma_unmap_addr_xxx APIs (looking at mlx5). Might
be worth mentioning that for the drivers managing the mappings
differently, care might be taken to not unmap netmems?

> +2. Driver should declare support by setting `netdev->netmem_tx = true`
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index fccc03cd2164..d8cfd5d69ddf 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1753,6 +1753,7 @@ enum netdev_reg_state {
>   *	@lltx:		device supports lockless Tx. Deprecated for real HW
>   *			drivers. Mainly used by logical interfaces, such as
>   *			bonding and tunnels
> + *	@netmem_tx:	device support netmem_tx.
>   *
>   *	@name:	This is the first field of the "visible" part of this structure
>   *		(i.e. as seen by users in the "Space.c" file).  It is the name
> @@ -2061,6 +2062,7 @@ struct net_device {
>  	struct_group(priv_flags_fast,
>  		unsigned long		priv_flags:32;
>  		unsigned long		lltx:1;
> +		unsigned long		netmem_tx:1;
>  	);
>  	const struct net_device_ops *netdev_ops;
>  	const struct header_ops *header_ops;
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index a2148ffb203d..1fb39ad63290 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -8,6 +8,7 @@
>  #ifndef _NET_NETMEM_H
>  #define _NET_NETMEM_H
>  
> +#include <linux/dma-mapping.h>
>  #include <linux/mm.h>
>  #include <net/net_debug.h>
>  
> @@ -267,4 +268,23 @@ static inline unsigned long netmem_get_dma_addr(netmem_ref netmem)
>  void get_netmem(netmem_ref netmem);
>  void put_netmem(netmem_ref netmem);
>  

[..]

> +#define netmem_dma_unmap_addr_set(NETMEM, PTR, ADDR_NAME, VAL)   \
> +	do {                                                     \
> +		if (!netmem_is_net_iov(NETMEM))                  \
> +			dma_unmap_addr_set(PTR, ADDR_NAME, VAL); \
> +		else                                             \
> +			dma_unmap_addr_set(PTR, ADDR_NAME, 0);   \
> +	} while (0)

Any reason not do to static inline instaed?