Re: CXL Namespaces of ACPI disappearing in Qemu demo

[Gregory Price]

On Tue, Aug 22, 2023 at 03:22:02PM +0800, Yuquan Wang wrote:
> Hi, Gregory
> I am sorry to disturb you, but there was still an ignored problem about CXL, 
> Although I had sent an email to jonathan (maybe he is busy recently so he 
> forgot to reply),  
> the Link is : 
> https://lists.nongnu.org/archive/html/qemu-arm/2023-08/msg00278.html 
> 
> Maybe the core question is that how should we use CXL components with a 
> standard PCIe system：
> 1) In Qemu, since the boards or machines in qemu can only use pxb-cxl 
> (attached on pcie.0) to add
> cxl host bridge, therefore, users should avoid to assign an occupied bus 
> number by other devices
> to pxb-cxl.
> 
> 2) In real hardware, CXL components should use independent CXL root/tree 
> (ACPI0017&ACPI0016)
> to separate from the namespace of default pcie root/domain.
> 
> I would be grateful if you have some free time to help check this issue : )
> 

Generally speaking, ACPI questions are outside the scope of my
knowledge, but to the extent that i have debugged QEMU for CXL I can say
that #1 is correct - the host bridge adapter should not share PCI bus
id's with anything else or you will have issues.  This can be
frustrating if you don't know what the magic numbers are.

for my work, i have been using this when i need to attach two pxb-cxl
deivces

qemu-system-x86_64 \
-drive file=./cxl.qcow2,format=qcow2,index=0,media=disk,id=hd \
-m 4G,slots=4,maxmem=8G \
-smp 4 \
-machine type=q35,cxl=on,hmat=on \
-device pxb-cxl,id=cxl.0,bus=pcie.0,bus_nr=52 \
-device pxb-cxl,id=cxl.1,bus=pcie.0,bus_nr=191 \


In regards to real hardware, I'm honestly not knowledgable enough about
ACPI or hardware to provide an intelligible answer unfortunately.

~Gregory