Merge branch 'device-memory-tcp'

Mina Almasry says:

====================
Device Memory TCP

Device memory TCP (devmem TCP) is a proposal for transferring data
to and/or from device memory efficiently, without bouncing the data
to a host memory buffer.

* Problem:

A large amount of data transfers have device memory as the source
and/or destination. Accelerators drastically increased the volume
of such transfers. Some examples include:

- ML accelerators transferring large amounts of training data from storage
  into GPU/TPU memory. In some cases ML training setup time can be as long
  as 50% of TPU compute time, improving data transfer throughput &
  efficiency can help improving GPU/TPU utilization.

- Distributed training, where ML accelerators, such as GPUs on different
  hosts, exchange data among them.

- Distributed raw block storage applications transfer large amounts of
  data with remote SSDs, much of this data does not require host
  processing.

Today, the majority of the Device-to-Device data transfers the network
are implemented as the following low level operations: Device-to-Host
copy, Host-to-Host network transfer, and Host-to-Device copy.

The implementation is suboptimal, especially for bulk data transfers,
and can put significant strains on system resources, such as host memory
bandwidth, PCIe bandwidth, etc. One important reason behind the current
state is the kernel’s lack of semantics to express device to network
transfers.

* Proposal:

In this patch series we attempt to optimize this use case by implementing
socket APIs that enable the user to:

1. send device memory across the network directly, and
2. receive incoming network packets directly into device memory.

Packet _payloads_ go directly from the NIC to device memory for receive
and from device memory to NIC for transmit.
Packet _headers_ go to/from host memory and are processed by the TCP/IP
stack normally. The NIC _must_ support header split to achieve this.

Advantages:

- Alleviate host memory bandwidth pressure, compared to existing
 network-transfer + device-copy semantics.

- Alleviate PCIe BW pressure, by limiting data transfer to the lowest level
  of the PCIe tree, compared to traditional path which sends data through
  the root complex.

* Patch overview:

** Part 1: netlink API

Gives user ability to bind dma-buf to an RX queue.

** Part 2: scatterlist support

Currently the standard for device memory sharing is DMABUF, which doesn't
generate struct pages. On the other hand, networking stack (skbs, drivers,
and page pool) operate on pages. We have 2 options:

1. Generate struct pages for dmabuf device memory, or,
2. Modify the networking stack to process scatterlist.

Approach #1 was attempted in RFC v1. RFC v2 implements approach #2.

** part 3: page pool support

We piggy back on page pool memory providers proposal:
https://github.com/kuba-moo/linux/tree/pp-providers

It allows the page pool to define a memory provider that provides the
page allocation and freeing. It helps abstract most of the device memory
TCP changes from the driver.

** part 4: support for unreadable skb frags

Page pool iovs are not accessible by the host; we implement changes
throughput the networking stack to correctly handle skbs with unreadable
frags.

** Part 5: recvmsg() APIs

We define user APIs for the user to send and receive device memory.

Not included with this series is the GVE devmem TCP support, just to
simplify the review. Code available here if desired:
https://github.com/mina/linux/tree/tcpdevmem

This series is built on top of net-next with Jakub's pp-providers changes
cherry-picked.

* NIC dependencies:

1. (strict) Devmem TCP require the NIC to support header split, i.e. the
   capability to split incoming packets into a header + payload and to put
   each into a separate buffer. Devmem TCP works by using device memory
   for the packet payload, and host memory for the packet headers.

2. (optional) Devmem TCP works better with flow steering support & RSS
   support, i.e. the NIC's ability to steer flows into certain rx queues.
   This allows the sysadmin to enable devmem TCP on a subset of the rx
   queues, and steer devmem TCP traffic onto these queues and non devmem
   TCP elsewhere.

The NIC I have access to with these properties is the GVE with DQO support
running in Google Cloud, but any NIC that supports these features would
suffice. I may be able to help reviewers bring up devmem TCP on their NICs.

* Testing:

The series includes a udmabuf kselftest that show a simple use case of
devmem TCP and validates the entire data path end to end without
a dependency on a specific dmabuf provider.

** Test Setup

Kernel: net-next with this series and memory provider API cherry-picked
locally.

Hardware: Google Cloud A3 VMs.

NIC: GVE with header split & RSS & flow steering support.
====================

Link: https://patch.msgid.link/20240910171458.219195-1-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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