docs/cxl: Add switch documentation

Switches were already introduced, but now we support them update
the documentation to provide an example in diagram and
qemu command line parameter forms.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20220616145126.8002-4-Jonathan.Cameron@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 86 insertions, 2 deletions

diff --git a/docs/system/devices/cxl.rst b/docs/system/devices/cxl.rst
index bcbfe8c490..a57e4c4e5c 100644
--- a/docs/system/devices/cxl.rst
+++ b/docs/system/devices/cxl.rst
@@ -118,8 +118,6 @@ and associated component register access via PCI bars.
 
 CXL Switch
 ~~~~~~~~~~
-Not yet implemented in QEMU.
-
 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
@@ -137,6 +135,10 @@ 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
@@ -240,6 +242,62 @@ Notes:
     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 1   |   |
+  |   | 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    |
+           |    appropiate 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 device::
@@ -279,6 +337,32 @@ the CXL Type3 device directly attached (no switches).::
   -device cxl-type3,bus=root_port16,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-aarch64 -M virt,gic-version=3,cxl=on -m 4g,maxmem=8G,slots=8 -cpu max \
+  ...
+  -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,memdev=cxl-mem0,lsa=cxl-lsa0,id=cxl-pmem0,size=256M \
+  -device cxl-downstream,port=1,bus=us0,id=swport1,chassis=0,slot=5 \
+  -device cxl-type3,bus=swport1,memdev=cxl-mem1,lsa=cxl-lsa1,id=cxl-pmem1,size=256M \
+  -device cxl-downstream,port=2,bus=us0,id=swport2,chassis=0,slot=6 \
+  -device cxl-type3,bus=swport2,memdev=cxl-mem2,lsa=cxl-lsa2,id=cxl-pmem2,size=256M \
+  -device cxl-downstream,port=3,bus=us0,id=swport3,chassis=0,slot=7 \
+  -device cxl-type3,bus=swport3,memdev=cxl-mem3,lsa=cxl-lsa3,id=cxl-pmem3,size=256M \
+  -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.size=4G,cxl-fmw.0.interleave-granularity=4k
+
 Kernel Configuration Options
 ----------------------------