7. NTB Rawdev Driver

The ntb rawdev driver provides a non-transparent bridge between two separate hosts so that they can communicate with each other. Thus, many user cases can benefit from this, such as fault tolerance and visual acceleration.

This PMD allows two hosts to handshake for device start and stop, memory allocation for the peer to access and read/write allocated memory from peer. Also, the PMD allows to use doorbell registers to notify the peer and share some information by using scratchpad registers.

7.1. BIOS setting on Intel Xeon

Intel Non-transparent Bridge (NTB) needs special BIOS settings on both systems. Note that for 4th Generation Intel® Xeon® Scalable Processors, option Port Subsystem Mode should be changed from Gen5 to Gen4 Only, then reboot.

Set Non-Transparent Bridge PCIe Port Definition for needed PCIe ports as NTB to NTB mode, on both hosts.
Set Enable NTB BARs as Enabled, on both hosts.
Set Enable SPLIT BARs as Disabled, on both hosts.
Set Imbar1 Size, Imbar2 Size, Embar1 Size and Embar2 Size, as 12-29 (i.e., 4K-512M) for 2nd Generation Intel® Xeon® Scalable Processors; as 12-51 (i.e., 4K-128PB) for 3rd and 4th Generation Intel® Xeon® Scalable Processors. Note that those bar sizes on both hosts should be the same.
Set Crosslink Control override as DSD/USP on one host, USD/DSP on another host.
Set PCIe PLL SSC (Spread Spectrum Clocking) as Disabled, on both hosts. This is a hardware requirement when using Re-timer Cards.

7.2. Device Setup

The Intel NTB devices need to be bound to a DPDK-supported kernel driver to use, i.e. igb_uio, vfio. The dpdk-devbind.py script can be used to show devices status and to bind them to a suitable kernel driver. They will appear under the category of “Misc (rawdev) devices”.

7.3. Prerequisites

NTB PMD needs kernel PCI driver to support write combining (WC) to get better performance. The difference will be more than 10 times. To enable WC, there are 2 ways.

Insert igb_uio with wc_activate=1 flag if use igb_uio driver.

insmod igb_uio.ko wc_activate=1

Enable WC for NTB device’s Bar 2 and Bar 4 (Mapped memory) manually. The reference is https://www.kernel.org/doc/html/latest/x86/mtrr.html Get bar base address using lspci -vvv -s ae:00.0 | grep Region.

# lspci -vvv -s ae:00.0 | grep Region
Region 0: Memory at 39bfe0000000 (64-bit, prefetchable) [size=64K]
Region 2: Memory at 39bfa0000000 (64-bit, prefetchable) [size=512M]
Region 4: Memory at 39bfc0000000 (64-bit, prefetchable) [size=512M]

Using the following command to enable WC.

echo "base=0x39bfa0000000 size=0x20000000 type=write-combining" >> /proc/mtrr
echo "base=0x39bfc0000000 size=0x20000000 type=write-combining" >> /proc/mtrr

And the results:

# cat /proc/mtrr
reg00: base=0x000000000 (    0MB), size= 2048MB, count=1: write-back
reg01: base=0x07f000000 ( 2032MB), size=   16MB, count=1: uncachable
reg02: base=0x39bfa0000000 (60553728MB), size=  512MB, count=1: write-combining
reg03: base=0x39bfc0000000 (60554240MB), size=  512MB, count=1: write-combining

To disable WC for these regions, using the following.

echo "disable=2" >> /proc/mtrr
echo "disable=3" >> /proc/mtrr

7.4. Ring Layout

Since read/write remote system’s memory are through PCI bus, remote read is much more expensive than remote write. Thus, the enqueue and dequeue based on ntb ring should avoid remote read. The ring layout for ntb is like the following:

Ring Format:

desc_ring:

   0               16                                              64
   +---------------------------------------------------------------+
   |                        buffer address                         |
   +---------------+-----------------------------------------------+
   | buffer length |                      resv                     |
   +---------------+-----------------------------------------------+

used_ring:

   0               16              32
   +---------------+---------------+
   | packet length |     flags     |
   +---------------+---------------+

Ring Layout:

+------------------------+   +------------------------+
| used_ring              |   | desc_ring              |
| +---+                  |   | +---+                  |
| |   |                  |   | |   |                  |
| +---+      +--------+  |   | +---+                  |
| |   | ---> | buffer | <+---+-|   |                  |
| +---+      +--------+  |   | +---+                  |
| |   |                  |   | |   |                  |
| +---+                  |   | +---+                  |
|  ...                   |   |  ...                   |
|                        |   |                        |
|            +---------+ |   |            +---------+ |
|            | tx_tail | |   |            | rx_tail | |
| System A   +---------+ |   | System B   +---------+ |
+------------------------+   +------------------------+
              <---------traffic---------

Enqueue and Dequeue Based on this ring layout, enqueue reads rx_tail to get how many free buffers and writes used_ring and tx_tail to tell the peer which buffers are filled with data. And dequeue reads tx_tail to get how many packets are arrived, and writes desc_ring and rx_tail to tell the peer about the new allocated buffers. So in this way, only remote write happens and remote read can be avoid to get better performance.

7.5. Limitation

This PMD is only supported on Intel Xeon Platforms:

4th Generation Intel® Xeon® Scalable Processors.
3rd Generation Intel® Xeon® Scalable Processors.
2nd Generation Intel® Xeon® Scalable Processors.