{"id":2230522,"url":"http://patchwork.ozlabs.org/api/1.1/patches/2230522/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/patch/20260429135717.3048713-2-joshualant@gmail.com/","project":{"id":14,"url":"http://patchwork.ozlabs.org/api/1.1/projects/14/?format=json","name":"QEMU Development","link_name":"qemu-devel","list_id":"qemu-devel.nongnu.org","list_email":"qemu-devel@nongnu.org","web_url":"","scm_url":"","webscm_url":""},"msgid":"<20260429135717.3048713-2-joshualant@gmail.com>","date":"2026-04-29T13:48:35","name":"[RFC,QEMU,01/10] docs: Add documentation for cxl-vcs-switch","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"c69ac6e8b247cc5eb5c566f8fe20415136acfbc9","submitter":{"id":93283,"url":"http://patchwork.ozlabs.org/api/1.1/people/93283/?format=json","name":"Joshua Lant","email":"joshualant@gmail.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/qemu-devel/patch/20260429135717.3048713-2-joshualant@gmail.com/mbox/","series":[{"id":502144,"url":"http://patchwork.ozlabs.org/api/1.1/series/502144/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/list/?series=502144","date":"2026-04-29T13:48:40","name":"Initial Support for VCS Switching","version":1,"mbox":"http://patchwork.ozlabs.org/series/502144/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2230522/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2230522/checks/","tags":{},"headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=pu3gZXaR;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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helo=mail-wr1-x42b.google.com","X-Spam_score_int":"-20","X-Spam_score":"-2.1","X-Spam_bar":"--","X-Spam_report":"(-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1,\n DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001,\n RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001,\n SPF_PASS=-0.001 autolearn=ham autolearn_force=no","X-Spam_action":"no action","X-Mailman-Approved-At":"Wed, 29 Apr 2026 16:03:14 -0400","X-BeenThere":"qemu-devel@nongnu.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"qemu development ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org"},"content":"Signed-off-by: Joshua Lant \n---\n docs/system/devices/cxl.rst | 90 ++++++++++++++++++++++++++++++++++---\n 1 file changed, 85 insertions(+), 5 deletions(-)","diff":"diff --git a/docs/system/devices/cxl.rst b/docs/system/devices/cxl.rst\nindex 32b1b5d773..9e8452e576 100644\n--- a/docs/system/devices/cxl.rst\n+++ b/docs/system/devices/cxl.rst\n@@ -119,11 +119,11 @@ and associated component register access via PCI bars.\n CXL Switch\n ~~~~~~~~~~\n Here we consider a simple CXL switch with only a single\n-virtual hierarchy. Whilst more complex devices exist, their\n-visibility to a particular host is generally the same as for\n-a simple switch design. Hosts often have no awareness\n-of complex rerouting and device pooling, they simply see\n-devices being hot added or hot removed.\n+virtual hierarchy. Whilst more complex devices exist (see VCS\n+Switching below), their visibility to a particular host is\n+generally the same as for a simple switch design. Hosts often\n+have no awareness of complex rerouting and device pooling,\n+they simply see devices being hot added or hot removed.\n \n A CXL switch has a similar architecture to those in PCIe,\n with a single upstream port, internal PCI bus and multiple\n@@ -467,6 +467,86 @@ Example configuration:\n Guest OS communication with the MCTP CCI can then be established using standard\n MCTP configuration tools.\n \n+CXL Multi-VCS Switching\n+-----------------------\n+\n+The cxl-vcs-switch object allows for a Fabric Manager to dynamically reconfigure\n+the switching within a multi-upstream port CXL/PCIe topology, This moves beyond\n+the static switching configuration described above. The use of vcs=X on an\n+endpoint device indicates that it should be hidden from guests at boot. Each\n+upstream port with vcs=X set will conceptually become an upstream PPB. Any\n+downstream port that is connected to an upstream port with vcs=X set will\n+automatically become a vPPB for that VCS. The overall cxl-virtual-switch has a\n+single CCI mailbox used for config/status of all ports within the switch.\n+Setting local-fm=true indicates that this QEMU instance has the CCI mailbox\n+attached. Setting it false will create listeners for commands from a remote\n+QEMU process (yet to be implemented).\n+\n+An example of how the topology is described on the CLI is shown below:\n+\n+ -object cxl-vcs-switch,id=vcs0,usp-ppbs=2,dsp-ppbs=4,local-fm=true \\\n+ -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.0,hdm_for_passthrough=true \\\n+ -device cxl-rp,port=0,bus=cxl.0,id=root_port1,chassis=0,slot=1 \\\n+ -device pxb-cxl,bus_nr=22,bus=pcie.0,id=cxl.1,hdm_for_passthrough=true \\\n+ -device cxl-rp,port=0,bus=cxl.1,id=root_port2,chassis=1,slot=1 \\\n+ -device cxl-upstream,port=0,sn=1234,bus=root_port1,id=us0,addr=0.0,multifunction=on,vcs=vcs0,usppb=0 \\\n+ -device cxl-upstream,port=0,sn=5678,bus=root_port2,id=us1,addr=0.0,multifunction=on,vcs=vcs0,usppb=1 \\\n+ -device cxl-switch-mailbox-cci,bus=root_port1,addr=0.3,target=vcs0 \\\n+ -device usb-cxl-mctp,bus=ehci.0,id=usb0,target=vcs0 \\\n+ -device cxl-downstream,port=0,bus=us0,id=dsp0,slot=3 \\\n+ -device cxl-downstream,port=1,bus=us0,id=dsp1,slot=4 \\\n+ -device cxl-downstream,port=0,bus=us1,id=dsp2,slot=7 \\\n+ -device cxl-downstream,port=1,bus=us1,id=dsp3,slot=8 \\\n+ -device cxl-type3,persistent-memdev=cxl-mem1,id=cxl-ep1,lsa=cxl-lsa1,sn=99,vcs=vcs0,dsppb=0 \\\n+ -device cxl-type3,persistent-memdev=cxl-mem2,id=cxl-ep2,lsa=cxl-lsa2,sn=100,vcs=vcs0,dsppb=1 \\\n+ -device cxl-type3,persistent-memdev=cxl-mem3,id=cxl-ep3,lsa=cxl-lsa3,sn=101,vcs=vcs0,dsppb=2 \\\n+ -device cxl-type3,persistent-memdev=cxl-mem4,id=cxl-ep4,lsa=cxl-lsa4,sn=102,vcs=vcs0,dsppb=3 \\\n+ -machine cxl-fmw.0.targets.0=cxl.0,cxl-fmw.0.size=8G,cxl-fmw.1.targets.0=cxl.1,cxl-fmw.1.size=8G\n+\n+Example topology involving VCS switching::\n+\n+ +--------------------+ +--------------------+\n+ | Host Bridge 0 | | Host Bridge 1 |\n+ +----------+---------+ +----------+---------+\n+ +-------+ | |\n+ | MCTP | | |\n+ | USB/ | +----------+---------+ +----------+---------+\n+ | I2C | | Root Port 0 | | Root Port 1 |\n+ +-----+-+ +----------+---------+ +----------+---------+\n+ | | |\n+ | | |\n+ +------|---------------+-----------------------+-----------------------+\n+ | +-+--------+ | cxl-vcs-switch (vcs0)| |\n+ | +--| CCI MBOX |---* | | |\n+ | | +----------+ | | |\n+ | | +-----------------+--------+ +-------+------------------+ |\n+ | +--+ | VCS0 | *---+ | VCS1 | |\n+ | | +---------------+------+ | | +-----+----------------+ | |\n+ | | | | | | | | | |\n+ | | | USP 0 | | | | USP 1 | | |\n+ | | | | | | | | | |\n+ | | +----+------------+----+ | | +----+------------+----+ | |\n+ | | | | | | | | | |\n+ | | +----+----+ +----+----+ | | +----+----+ +----+----+ | |\n+ | | | DSP 0 | | DSP 1 | | | | DSP 2 | | DSP 3 | | |\n+ | | |(vPPB 0) | |(vPPB 1) | | | |(vPPB 0) | |(vPPB 1) | | |\n+ | | | | | | | | | | | | | |\n+ | | +---------+ +---------+ | | +---------+ +----+----+ | |\n+ | +--------------------------+ +-------------------+------+ |\n+ | | |\n+ | +----------------------------------------------+ |\n+ | | |\n+ | | - - - |\n+ +-----------|------------|--------------------|------------|-----------+\n+ | | | |\n+ +---------+ +---------+ +---------+ +---------+\n+ |CXL/PCIe | |CXL/PCIe | |CXL/PCIe | |CXL/PCIe |\n+ | EP 0 | | EP 1 | | EP 2 | | EP 3 |\n+ | (PPB0) | | (PPB1) | | (PPB2) | | (PPB3) |\n+ +---------+ +---------+ +---------+ +---------+\n+ PPB0 Bound to VCS1, vPPB1. Others unbound...\n+\n+\n References\n ----------\n \n","prefixes":["RFC","QEMU","01/10"]}