AMD's CTO, Mark Papermaster, has stated in a blog that AMD has joined the Compute Express Link (CXL) Consortium. The industry group is led by nine industry giants including Intel, Alibaba, Google, and Microsoft, but has over 35 members. The CXL 1.0 technology uses the PCIe 5.0 physical infrastructure to enable a coherent low-latency interconnect protocol that allows to share CPU and non-CPU resources efficiently and without using complex memory management. The announcement indicates that AMD now supports all of the current and upcoming non-proprietary high-speed interconnect protocols, including CCIX, Gen-Z, and OpenCAPI.

PCIe has enabled a tremendous increase of bandwidth from 2.5 GT/s per lane in 2003 to 32 GT/s per lane in 2019 and is set to remain a ubiquitous physical interface of upcoming SoCs. Over the past few years it turned out that to enable efficient coherent interconnect between CPUs and other devices, specific low-latency protocols were needed, so a variety of proprietary and open-standard technologies built upon PCIe PHY were developed, including CXL, CCIX, Gen-Z, Infinity Fabric, NVLink, CAPI, and other. In 2016, IBM (with a group of supporters) went as far as developing the OpenCAPI interface relying on a new physical layer and a new protocol (but this is a completely different story).

Each of the protocols that rely on PCIe have their peculiarities and numerous supporters. The CXL 1.0 specification introduced earlier this year was primarily designed to enable heterogeneous processing (by using accelerators) and memory systems (think memory expansion devices). The low-latency CXL runs on PCIe 5.0 PHY stack at 32 GT/s and supports x16, x8, and x4 link widths natively. Meanwhile, in degraded mode it also supports 16.0 GT/s and 8.0 GT/s data rates as well as x2 and x1 links. In case of a PCIe 5.0 x16 slot, CXL 1.0 devices will enjoy 64 GB/s bandwidth in each direction. It is also noteworthy that the CXL 1.0 features three protocols within itself: the mandatory CXL.io as well as CXL.cache for cache coherency and CXL.memory for memory coherency that are needed to effectively manage latencies.

In the coming years computers in general and machines used for AI and ML processing will require a diverse combination of accelerators featuring scalar, vector, matrix and spatial architectures. For efficient operation, some of these accelerators will need to have low-latency cache coherency and memory semantics between them and processors, but since there is no ubiquitous protocol that supports appropriate functionality, there will be a fight between some of the standards that do not complement each other.

The biggest advantage of CXL is that it is not only supported by over 30 companies already, but its founding members include such heavyweights as Alibaba, DellEMC, Facebook, Google, HPE, Huawei, Intel, and Microsoft. All of these companies build their own hardware architectures and their support for CXL means that they plan to use the technology. Since AMD clearly does not want to be left behind the industry, it is natural for the company to join the CXL party.

Since CXL relies on PCIe 5.0 physical infrastructure, companies can use the same physical interconnects but develop the transmission logic required. At this point AMD is not committing to enabling CXL on future products, but is throwing its hat into the ring to discuss how the protocol develops, should it appear in a future AMD product.

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Sources: AMD, CXL Consortium, PLDA