PCIe RC\EP virtio rdma solution discussion.

[Frank Li <Frank.Li@nxp.com>]

From: Frank Li <Frank.li@nxp.com>

Recently more and more people are interested in PCI RC and EP connection,
especially network usage cases. I upstreamed a vntb solution last year. 
But the transfer speed is not good enough. I initialized a discussion 
at https://lore.kernel.org/imx/d098a631-9930-26d3-48f3-8f95386c8e50@ti.com/T/#t
 
  ┌─────────────────────────────────┐   ┌──────────────┐
  │                                 │   │              │
  │                                 │   │              │
  │   VirtQueue             RX      │   │  VirtQueue   │
  │     TX                 ┌──┐     │   │    TX        │
  │  ┌─────────┐           │  │     │   │ ┌─────────┐  │
  │  │ SRC LEN ├─────┐  ┌──┤  │◄────┼───┼─┤ SRC LEN │  │
  │  ├─────────┤     │  │  │  │     │   │ ├─────────┤  │
  │  │         │     │  │  │  │     │   │ │         │  │
  │  ├─────────┤     │  │  │  │     │   │ ├─────────┤  │
  │  │         │     │  │  │  │     │   │ │         │  │
  │  └─────────┘     │  │  └──┘     │   │ └─────────┘  │
  │                  │  │           │   │              │
  │     RX       ┌───┼──┘   TX      │   │    RX        │
  │  ┌─────────┐ │   │     ┌──┐     │   │ ┌─────────┐  │
  │  │         │◄┘   └────►│  ├─────┼───┼─┤         │  │
  │  ├─────────┤           │  │     │   │ ├─────────┤  │
  │  │         │           │  │     │   │ │         │  │
  │  ├─────────┤           │  │     │   │ ├─────────┤  │
  │  │         │           │  │     │   │ │         │  │
  │  └─────────┘           │  │     │   │ └─────────┘  │
  │   virtio_net           └──┘     │   │ virtio_net   │
  │  Virtual PCI BUS   EDMA Queue   │   │              │
  ├─────────────────────────────────┤   │              │
  │  PCI EP Controller with eDMA    │   │  PCI Host    │
  └─────────────────────────────────┘   └──────────────┘

Basic idea is
	1.	Both EP and host probe virtio_net driver
	2.	There are two queues,  one is the EP side(EQ),  the other is the Host side. 
	3.	EP side epf driver map Host side’s queue into EP’s space. Called HQ.
	4.	One working thread 
	5.	pick one TX from EQ and RX from HQ, combine and generate EDMA requests, 
and put into the DMA TX queue.
	6.	Pick one RX from EQ and TX from HQ, combine and generate EDMA requests,
and put into the DMA RX queue. 
	7.	EDMA done irq will mark related item in EP and HQ finished.

The whole transfer is zero copied and uses a DMA queue.

The Shunsuke Mie implemented the above idea. 
 https://lore.kernel.org/linux-pci/CANXvt5q_qgLuAfF7dxxrqUirT_Ld4B=POCq8JcB9uPRvCGDiKg@mail.gmail.com/T/#t


Similar solution posted at 2019, except use memcpy from/to PCI EP map windows. 
Using DMA should be simpler because EDMA can access the whole HOST\EP side memory space. 
https://lore.kernel.org/linux-pci/9f8e596f-b601-7f97-a98a-111763f966d1@ti.com/T/

Solution 1 (Based on shunsuke):

Both EP and Host side use virtio.
Using EDMA to simplify data transfer and improve transfer speed.
RDMA implement based on RoCE
- proposal: https://lore.kernel.org/all/20220511095900.343-1-xieyongji@bytedance.com/T/
- presentation on kvm forum: https://youtu.be/Qrhv6hC_YK4

Solution 2(2020, Kishon)

Previous https://lore.kernel.org/linux-pci/20200702082143.25259-1-kishon@ti.com/
EP side use vhost, RC side use virtio.
I don’t think anyone works on this thread now.
If using eDMA, it needs both sides to have a transfer queue. 
I don't know how to easily implement it on the vhost side. 

Solution 3(I am working on)

Implement infiniband rdma driver at both EP and RC side. 
EP side build EDMA hardware queue based on EP/RC side’s send and receive
queue and when eDMA finished, write status to complete queue for both EP/RC 
side. Use ipoib implement network transfer.


The whole upstream effort is quite huge for these. I don’t want to waste
time and efforts because direction is wrong. 

I think Solution 1 is an easy path.