| Compute Express Link (CXL) |
| ========================== |
| From the view of a single host, CXL is an interconnect standard that |
| targets accelerators and memory devices attached to a CXL host. |
| This description will focus on those aspects visible either to |
| software running on a QEMU emulated host or to the internals of |
| functional emulation. As such, it will skip over many of the |
| electrical and protocol elements that would be more of interest |
| for real hardware and will dominate more general introductions to CXL. |
| It will also completely ignore the fabric management aspects of CXL |
| by considering only a single host and a static configuration. |
| |
| CXL shares many concepts and much of the infrastructure of PCI Express, |
| with CXL Host Bridges, which have CXL Root Ports which may be directly |
| attached to CXL or PCI End Points. Alternatively there may be CXL Switches |
| with CXL and PCI Endpoints attached below them. In many cases additional |
| control and capabilities are exposed via PCI Express interfaces. |
| This sharing of interfaces and hence emulation code is reflected |
| in how the devices are emulated in QEMU. In most cases the various |
| CXL elements are built upon an equivalent PCIe devices. |
| |
| CXL devices support the following interfaces: |
| |
| * Most conventional PCIe interfaces |
| |
| - Configuration space access |
| - BAR mapped memory accesses used for registers and mailboxes. |
| - MSI/MSI-X |
| - AER |
| - DOE mailboxes |
| - IDE |
| - Many other PCI express defined interfaces.. |
| |
| * Memory operations |
| |
| - Equivalent of accessing DRAM / NVDIMMs. Any access / feature |
| supported by the host for normal memory should also work for |
| CXL attached memory devices. |
| |
| * Cache operations. The are mostly irrelevant to QEMU emulation as |
| QEMU is not emulating a coherency protocol. Any emulation related |
| to these will be device specific and is out of the scope of this |
| document. |
| |
| CXL 2.0 Device Types |
| -------------------- |
| CXL 2.0 End Points are often categorized into three types. |
| |
| **Type 1:** These support coherent caching of host memory. Example might |
| be a crypto accelerators. May also have device private memory accessible |
| via means such as PCI memory reads and writes to BARs. |
| |
| **Type 2:** These support coherent caching of host memory and host |
| managed device memory (HDM) for which the coherency protocol is managed |
| by the host. This is a complex topic, so for more information on CXL |
| coherency see the CXL 2.0 specification. |
| |
| **Type 3 Memory devices:** These devices act as a means of attaching |
| additional memory (HDM) to a CXL host including both volatile and |
| persistent memory. The CXL topology may support interleaving across a |
| number of Type 3 memory devices using HDM Decoders in the host, host |
| bridge, switch upstream port and endpoints. |
| |
| Scope of CXL emulation in QEMU |
| ------------------------------ |
| The focus of CXL emulation is CXL revision 2.0 and later. Earlier CXL |
| revisions defined a smaller set of features, leaving much of the control |
| interface as implementation defined or device specific, making generic |
| emulation challenging with host specific firmware being responsible |
| for setup and the Endpoints being presented to operating systems |
| as Root Complex Integrated End Points. CXL rev 2.0 looks a lot |
| more like PCI Express, with fully specified discoverability |
| of the CXL topology. |
| |
| CXL System components |
| ---------------------- |
| A CXL system is made up a Host with a number of 'standard components' |
| the control and capabilities of which are discoverable by system software |
| using means described in the CXL 2.0 specification. |
| |
| CXL Fixed Memory Windows (CFMW) |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| A CFMW consists of a particular range of Host Physical Address space |
| which is routed to particular CXL Host Bridges. At time of generic |
| software initialization it will have a particularly interleaving |
| configuration and associated Quality of Service Throttling Group (QTG). |
| This information is available to system software, when making |
| decisions about how to configure interleave across available CXL |
| memory devices. It is provide as CFMW Structures (CFMWS) in |
| the CXL Early Discovery Table, an ACPI table. |
| |
| Note: QTG 0 is the only one currently supported in QEMU. |
| |
| CXL Host Bridge (CXL HB) |
| ~~~~~~~~~~~~~~~~~~~~~~~~ |
| A CXL host bridge is similar to the PCIe equivalent, but with a |
| specification defined register interface called CXL Host Bridge |
| Component Registers (CHBCR). The location of this CHBCR MMIO |
| space is described to system software via a CXL Host Bridge |
| Structure (CHBS) in the CEDT ACPI table. The actual interfaces |
| are identical to those used for other parts of the CXL hierarchy |
| as CXL Component Registers in PCI BARs. |
| |
| Interfaces provided include: |
| |
| * Configuration of HDM Decoders to route CXL Memory accesses with |
| a particularly Host Physical Address range to the target port |
| below which the CXL device servicing that address lies. This |
| may be a mapping to a single Root Port (RP) or across a set of |
| target RPs. |
| |
| CXL Root Ports (CXL RP) |
| ~~~~~~~~~~~~~~~~~~~~~~~ |
| A CXL Root Port serves the same purpose as a PCIe Root Port. |
| There are a number of CXL specific Designated Vendor Specific |
| Extended Capabilities (DVSEC) in PCIe Configuration Space |
| and associated component register access via PCI bars. |
| |
| CXL Switch |
| ~~~~~~~~~~ |
| Here we consider a simple CXL switch with only a single |
| virtual hierarchy. Whilst more complex devices exist, their |
| visibility to a particular host is generally the same as for |
| a simple switch design. Hosts often have no awareness |
| of complex rerouting and device pooling, they simply see |
| devices being hot added or hot removed. |
| |
| A CXL switch has a similar architecture to those in PCIe, |
| with a single upstream port, internal PCI bus and multiple |
| downstream ports. |
| |
| Both the CXL upstream and downstream ports have CXL specific |
| DVSECs in configuration space, and component registers in PCI |
| BARs. The Upstream Port has the configuration interfaces for |
| the HDM decoders which route incoming memory accesses to the |
| appropriate downstream port. |
| |
| A CXL switch is created in a similar fashion to PCI switches |
| by creating an upstream port (cxl-upstream) and a number of |
| downstream ports on the internal switch bus (cxl-downstream). |
| |
| CXL Memory Devices - Type 3 |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| CXL type 3 devices use a PCI class code and are intended to be supported |
| by a generic operating system driver. They have HDM decoders |
| though in these EP devices, the decoder is responsible not for |
| routing but for translation of the incoming host physical address (HPA) |
| into a Device Physical Address (DPA). |
| |
| CXL Memory Interleave |
| --------------------- |
| To understand the interaction of different CXL hardware components which |
| are emulated in QEMU, let us consider a memory read in a fully configured |
| CXL topology. Note that system software is responsible for configuration |
| of all components with the exception of the CFMWs. System software is |
| responsible for allocating appropriate ranges from within the CFMWs |
| and exposing those via normal memory configurations as would be done |
| for system RAM. |
| |
| Example system Topology. x marks the match in each decoder level:: |
| |
| |<------------------SYSTEM PHYSICAL ADDRESS MAP (1)----------------->| |
| | __________ __________________________________ __________ | |
| | | | | | | | | |
| | | CFMW 0 | | CXL Fixed Memory Window 1 | | CFMW 2 | | |
| | | HB0 only | | Configured to interleave memory | | HB1 only | | |
| | | | | memory accesses across HB0/HB1 | | | | |
| | |__________| |_____x____________________________| |__________| | |
| | | | | |
| | | | | |
| | | | | |
| | Interleave Decoder | | |
| | Matches this HB | | |
| \_____________| |_____________/ |
| __________|__________ _____|_______________ |
| | | | | |
| (2) | CXL HB 0 | | CXL HB 1 | |
| | HB IntLv Decoders | | HB IntLv Decoders | |
| | PCI/CXL Root Bus 0c | | PCI/CXL Root Bus 0d | |
| | | | | |
| |___x_________________| |_____________________| |
| | | | | |
| | | | | |
| A HB 0 HDM Decoder | | | |
| matches this Port | | | |
| | | | | |
| ___________|___ __________|__ __|_________ ___|_________ |
| (3)| Root Port 0 | | Root Port 1 | | Root Port 2| | Root Port 3 | |
| | Appears in | | Appears in | | Appears in | | Appear in | |
| | PCI topology | | PCI Topology| | PCI Topo | | PCI Topo | |
| | As 0c:00.0 | | as 0c:01.0 | | as de:00.0 | | as de:01.0 | |
| |_______________| |_____________| |____________| |_____________| |
| | | | | |
| | | | | |
| _____|_________ ______|______ ______|_____ ______|_______ |
| (4)| x | | | | | | | |
| | CXL Type3 0 | | CXL Type3 1 | | CXL type3 2| | CLX Type 3 3 | |
| | | | | | | | | |
| | PMEM0(Vol LSA)| | PMEM1 (...) | | PMEM2 (...)| | PMEM3 (...) | |
| | Decoder to go | | | | | | | |
| | from host PA | | PCI 0e:00.0 | | PCI df:00.0| | PCI e0:00.0 | |
| | to device PA | | | | | | | |
| | PCI as 0d:00.0| | | | | | | |
| |_______________| |_____________| |____________| |______________| |
| |
| Notes: |
| |
| (1) **3 CXL Fixed Memory Windows (CFMW)** corresponding to different |
| ranges of the system physical address map. Each CFMW has |
| particular interleave setup across the CXL Host Bridges (HB) |
| CFMW0 provides uninterleaved access to HB0, CFMW2 provides |
| uninterleaved access to HB1. CFMW1 provides interleaved memory access |
| across HB0 and HB1. |
| |
| (2) **Two CXL Host Bridges**. Each of these has 2 CXL Root Ports and |
| programmable HDM decoders to route memory accesses either to |
| a single port or interleave them across multiple ports. |
| A complex configuration here, might be to use the following HDM |
| decoders in HB0. HDM0 routes CFMW0 requests to RP0 and hence |
| part of CXL Type3 0. HDM1 routes CFMW0 requests from a |
| different region of the CFMW0 PA range to RP2 and hence part |
| of CXL Type 3 1. HDM2 routes yet another PA range from within |
| CFMW0 to be interleaved across RP0 and RP1, providing 2 way |
| interleave of part of the memory provided by CXL Type3 0 and |
| CXL Type 3 1. HDM3 routes those interleaved accesses from |
| CFMW1 that target HB0 to RP 0 and another part of the memory of |
| CXL Type 3 0 (as part of a 2 way interleave at the system level |
| across for example CXL Type3 0 and CXL Type3 2. |
| HDM4 is used to enable system wide 4 way interleave across all |
| the present CXL type3 devices, by interleaving those (interleaved) |
| requests that HB0 receives from from CFMW1 across RP 0 and |
| RP 1 and hence to yet more regions of the memory of the |
| attached Type3 devices. Note this is a representative subset |
| of the full range of possible HDM decoder configurations in this |
| topology. |
| |
| (3) **Four CXL Root Ports.** In this case the CXL Type 3 devices are |
| directly attached to these ports. |
| |
| (4) **Four CXL Type3 memory expansion devices.** These will each have |
| HDM decoders, but in this case rather than performing interleave |
| they will take the Host Physical Addresses of accesses and map |
| them to their own local Device Physical Address Space (DPA). |
| |
| Example topology involving a switch:: |
| |
| |<------------------SYSTEM PHYSICAL ADDRESS MAP (1)----------------->| |
| | __________ __________________________________ __________ | |
| | | | | | | | | |
| | | CFMW 0 | | CXL Fixed Memory Window 1 | | CFMW 2 | | |
| | | HB0 only | | Configured to interleave memory | | HB1 only | | |
| | | | | memory accesses across HB0/HB1 | | | | |
| | |____x_____| |__________________________________| |__________| | |
| | | | | |
| | | | | |
| | | | |
| Interleave Decoder | | | |
| Matches this HB | | | |
| \_____________| |_____________/ |
| __________|__________ _____|_______________ |
| | | | | |
| | CXL HB 0 | | CXL HB 1 | |
| | HB IntLv Decoders | | HB IntLv Decoders | |
| | PCI/CXL Root Bus 0c | | PCI/CXL Root Bus 0d | |
| | | | | |
| |___x_________________| |_____________________| |
| | | | | |
| | |
| A HB 0 HDM Decoder |
| matches this Port |
| ___________|___ |
| | Root Port 0 | |
| | Appears in | |
| | PCI topology | |
| | As 0c:00.0 | |
| |___________x___| |
| | |
| | |
| \_____________________ |
| | |
| | |
| --------------------------------------------------- |
| | Switch 0 USP as PCI 0d:00.0 | |
| | USP has HDM decoder which direct traffic to | |
| | appropriate downstream port | |
| | Switch BUS appears as 0e | |
| |x__________________________________________________| |
| | | | | |
| | | | | |
| _____|_________ ______|______ ______|_____ ______|_______ |
| (4)| x | | | | | | | |
| | CXL Type3 0 | | CXL Type3 1 | | CXL type3 2| | CLX Type 3 3 | |
| | | | | | | | | |
| | PMEM0(Vol LSA)| | PMEM1 (...) | | PMEM2 (...)| | PMEM3 (...) | |
| | Decoder to go | | | | | | | |
| | from host PA | | PCI 10:00.0 | | PCI 11:00.0| | PCI 12:00.0 | |
| | to device PA | | | | | | | |
| | PCI as 0f:00.0| | | | | | | |
| |_______________| |_____________| |____________| |______________| |
| |
| Example command lines |
| --------------------- |
| A very simple setup with just one directly attached CXL Type 3 Persistent Memory device:: |
| |
| qemu-system-x86_64 -M q35,cxl=on -m 4G,maxmem=8G,slots=8 -smp 4 \ |
| ... |
| -object memory-backend-file,id=cxl-mem1,share=on,mem-path=/tmp/cxltest.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa1,share=on,mem-path=/tmp/lsa.raw,size=256M \ |
| -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.1 \ |
| -device cxl-rp,port=0,bus=cxl.1,id=root_port13,chassis=0,slot=2 \ |
| -device cxl-type3,bus=root_port13,persistent-memdev=cxl-mem1,lsa=cxl-lsa1,id=cxl-pmem0 \ |
| -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.size=4G |
| |
| A very simple setup with just one directly attached CXL Type 3 Volatile Memory device:: |
| |
| qemu-system-aarch64 -M virt,gic-version=3,cxl=on -m 4g,maxmem=8G,slots=8 -cpu max \ |
| ... |
| -object memory-backend-ram,id=vmem0,share=on,size=256M \ |
| -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.1 \ |
| -device cxl-rp,port=0,bus=cxl.1,id=root_port13,chassis=0,slot=2 \ |
| -device cxl-type3,bus=root_port13,volatile-memdev=vmem0,id=cxl-vmem0 \ |
| -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.size=4G |
| |
| The same volatile setup may optionally include an LSA region:: |
| |
| qemu-system-aarch64 -M virt,gic-version=3,cxl=on -m 4g,maxmem=8G,slots=8 -cpu max \ |
| ... |
| -object memory-backend-ram,id=vmem0,share=on,size=256M \ |
| -object memory-backend-file,id=cxl-lsa0,share=on,mem-path=/tmp/lsa.raw,size=256M \ |
| -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.1 \ |
| -device cxl-rp,port=0,bus=cxl.1,id=root_port13,chassis=0,slot=2 \ |
| -device cxl-type3,bus=root_port13,volatile-memdev=vmem0,lsa=cxl-lsa0,id=cxl-vmem0 \ |
| -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.size=4G |
| |
| A setup suitable for 4 way interleave. Only one fixed window provided, to enable 2 way |
| interleave across 2 CXL host bridges. Each host bridge has 2 CXL Root Ports, with |
| the CXL Type3 device directly attached (no switches).:: |
| |
| qemu-system-x86_64 -M q35,cxl=on -m 4G,maxmem=8G,slots=8 -smp 4 \ |
| ... |
| -object memory-backend-file,id=cxl-mem1,share=on,mem-path=/tmp/cxltest.raw,size=256M \ |
| -object memory-backend-file,id=cxl-mem2,share=on,mem-path=/tmp/cxltest2.raw,size=256M \ |
| -object memory-backend-file,id=cxl-mem3,share=on,mem-path=/tmp/cxltest3.raw,size=256M \ |
| -object memory-backend-file,id=cxl-mem4,share=on,mem-path=/tmp/cxltest4.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa1,share=on,mem-path=/tmp/lsa.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa2,share=on,mem-path=/tmp/lsa2.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa3,share=on,mem-path=/tmp/lsa3.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa4,share=on,mem-path=/tmp/lsa4.raw,size=256M \ |
| -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.1 \ |
| -device pxb-cxl,bus_nr=222,bus=pcie.0,id=cxl.2 \ |
| -device cxl-rp,port=0,bus=cxl.1,id=root_port13,chassis=0,slot=2 \ |
| -device cxl-type3,bus=root_port13,persistent-memdev=cxl-mem1,lsa=cxl-lsa1,id=cxl-pmem0 \ |
| -device cxl-rp,port=1,bus=cxl.1,id=root_port14,chassis=0,slot=3 \ |
| -device cxl-type3,bus=root_port14,persistent-memdev=cxl-mem2,lsa=cxl-lsa2,id=cxl-pmem1 \ |
| -device cxl-rp,port=0,bus=cxl.2,id=root_port15,chassis=0,slot=5 \ |
| -device cxl-type3,bus=root_port15,persistent-memdev=cxl-mem3,lsa=cxl-lsa3,id=cxl-pmem2 \ |
| -device cxl-rp,port=1,bus=cxl.2,id=root_port16,chassis=0,slot=6 \ |
| -device cxl-type3,bus=root_port16,persistent-memdev=cxl-mem4,lsa=cxl-lsa4,id=cxl-pmem3 \ |
| -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.targets.1=cxl.2,cxl-fmw.0.size=4G,cxl-fmw.0.interleave-granularity=8k |
| |
| An example of 4 devices below a switch suitable for 1, 2 or 4 way interleave:: |
| |
| qemu-system-x86_64 -M q35,cxl=on -m 4G,maxmem=8G,slots=8 -smp 4 \ |
| ... |
| -object memory-backend-file,id=cxl-mem0,share=on,mem-path=/tmp/cxltest.raw,size=256M \ |
| -object memory-backend-file,id=cxl-mem1,share=on,mem-path=/tmp/cxltest1.raw,size=256M \ |
| -object memory-backend-file,id=cxl-mem2,share=on,mem-path=/tmp/cxltest2.raw,size=256M \ |
| -object memory-backend-file,id=cxl-mem3,share=on,mem-path=/tmp/cxltest3.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa0,share=on,mem-path=/tmp/lsa0.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa1,share=on,mem-path=/tmp/lsa1.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa2,share=on,mem-path=/tmp/lsa2.raw,size=256M \ |
| -object memory-backend-file,id=cxl-lsa3,share=on,mem-path=/tmp/lsa3.raw,size=256M \ |
| -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.1 \ |
| -device cxl-rp,port=0,bus=cxl.1,id=root_port0,chassis=0,slot=0 \ |
| -device cxl-rp,port=1,bus=cxl.1,id=root_port1,chassis=0,slot=1 \ |
| -device cxl-upstream,bus=root_port0,id=us0 \ |
| -device cxl-downstream,port=0,bus=us0,id=swport0,chassis=0,slot=4 \ |
| -device cxl-type3,bus=swport0,persistent-memdev=cxl-mem0,lsa=cxl-lsa0,id=cxl-pmem0 \ |
| -device cxl-downstream,port=1,bus=us0,id=swport1,chassis=0,slot=5 \ |
| -device cxl-type3,bus=swport1,persistent-memdev=cxl-mem1,lsa=cxl-lsa1,id=cxl-pmem1 \ |
| -device cxl-downstream,port=2,bus=us0,id=swport2,chassis=0,slot=6 \ |
| -device cxl-type3,bus=swport2,persistent-memdev=cxl-mem2,lsa=cxl-lsa2,id=cxl-pmem2 \ |
| -device cxl-downstream,port=3,bus=us0,id=swport3,chassis=0,slot=7 \ |
| -device cxl-type3,bus=swport3,persistent-memdev=cxl-mem3,lsa=cxl-lsa3,id=cxl-pmem3 \ |
| -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.size=4G,cxl-fmw.0.interleave-granularity=4k |
| |
| Deprecations |
| ------------ |
| |
| The Type 3 device [memdev] attribute has been deprecated in favor of the |
| [persistent-memdev] attributes. [memdev] will default to a persistent memory |
| device for backward compatibility and is incapable of being used in combination |
| with [persistent-memdev]. |
| |
| Kernel Configuration Options |
| ---------------------------- |
| |
| In Linux 5.18 the following options are necessary to make use of |
| OS management of CXL memory devices as described here. |
| |
| * CONFIG_CXL_BUS |
| * CONFIG_CXL_PCI |
| * CONFIG_CXL_ACPI |
| * CONFIG_CXL_PMEM |
| * CONFIG_CXL_MEM |
| * CONFIG_CXL_PORT |
| * CONFIG_CXL_REGION |
| |
| References |
| ---------- |
| |
| - Consortium website for specifications etc: |
| http://www.computeexpresslink.org |
| - Compute Express link Revision 2 specification, October 2020 |
| - CEDT CFMWS & QTG _DSM ECN May 2021 |
