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

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 21 Feb 2025 16:35:24 -0800

On 02/21, Mina Almasry wrote:
> On Thu, Feb 20, 2025 at 11:13 AM Stanislav Fomichev
> <[email-masked]> wrote:
> >
> > 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?
> >
> 
> Yes now that I take a closer look, it's poorly worded to imply the
> issue is limited to dma_unmap. I will reword to say that all
> dma_map*() APIs must be avoided, and we have helpers for
> dma_unmap_*(), and more helpers can be added if needed (similar to
> wording in the Driver RX requirements).
> 
> > > +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?
> 
> Because the args passed to dma_unmap_addr_set are quite unique,
> AFAICT. PTR is a pointer to any struct that has a field (anywhere)
> inside of it called ADDR_NAME, then dma_unmap_addr_set does something
> like:
> 
> PTR->ADDR_NAME = VAL;
> 
> A static inline needs well defined types, and I couldn't figure out
> how to do that (or if it is possible), so a macro it is I guess.
> 
> Where I could, I went with static inline.

Ah dma_unmap_addr_set itself is an ugly define :-( Makes sense.