Re: [PATCH v13 0/5] arm/virt: CXL support via pxb_cxl

[Itaru Kitayama <itaru.kitayama@linux.dev>]

Hi Jonathan,

> On May 13, 2025, at 20:14, Jonathan Cameron <Jonathan.Cameron@huawei.com> wrote:
> 
> V13:
> - Make CXL fixed memory windows sysbus devices.
>  IIRC this was requested by Peter in one of the reviews a long time back
>  but at the time the motivation was less strong than it becomes with some
>  WiP patches for hotness monitoring and high performance direct connect
>  where we need a machine type independent way to iterate all the CXL
>  fixed memory windows. This is a convenient place to do it so drag that
>  work forward into this series.
> 
>  This allows us to drop separate list and necessary machine specific
>  access code in favour of
>  object_child_foreach_recursive(object_get_root(),...)
>  One snag is that the ordering of multiple fixed memory windows in that
>  walk depends on the underlying g_hash_table iterations rather than the
>  order of creation. In the memory map layout and ACPI table creation we
>  need both stable and predictable ordering. Resolve this in a similar
>  fashion to object_class_get_list_sorted() be throwing them in a GSList
>  and sorting that. Only use this when a sorted list is needed.
> 
>  Dropped RFC as now I'm happy with this code and would like to get it
>  upstream!  Particularly as it broken even today due to enscripten
>  related changes that stop us using g_slist_sort(). Easy fix though.
> 
> Note that we have an issue for CXL emulation in general and TCG which
> is being discussed in:
> https://lore.kernel.org/all/20250425183524.00000b28@huawei.com/
> (also affects some other platforms)
> 
> Until that is resolved, either rebase this back on 10.0 or just
> don't let code run out of it (don't use KMEM to expose it as normal
> memory, use DAX instead).
> 
> Previous cover letter.
> 
> Back in 2022, this series stalled on the absence of a solution to device
> tree support for PCI Expander Bridges (PXB) and we ended up only having
> x86 support upstream. I've been carrying the arm64 support out of tree
> since then, with occasional nasty surprises (e.g. UNIMP + DT issue seen
> a few weeks ago) and a fair number of fiddly rebases.
> gitlab.com/jic23/qemu cxl-<latest date>
> 
> A recent discussion with Peter Maydell indicated that there are various
> other ACPI only features now, so in general he might be more relaxed
> about DT support being necessary. The upcoming vSMMUv3 support would
> run into this problem as well.
> 
> I presented the background to the PXB issue at Linaro connect 2022. In
> short the issue is that PXBs steal MMIO space from the main PCI root
> bridge. The challenge is knowing how much to steal.
> 
> On ACPI platforms, we can rely on EDK2 to perform an enumeration and
> configuration of the PCI topology and QEMU can update the ACPI tables
> after EDK2 has done this when it can simply read the space used by the
> root ports. On device tree, there is no entity to figure out that
> enumeration so we don't know how to size the stolen region.
> 
> Three approaches were discussed:
> 1) Enumerating in QEMU. Horribly complex and the last thing we want is a
>   3rd enumeration implementation that ends up out of sync with EDK2 and
>   the kernel (there are frequent issues because of how those existing
>   implementations differ.
> 2) Figure out how to enumerate in kernel. I never put a huge amount of work
>   into this, but it seemed likely to involve a nasty dance with similar
>   very specific code to that EDK2 is carrying and would very challenging
>   to upstream (given the lack of clarity on real use cases for PXBs and
>   DT).
> 3) Hack it based on the control we have which is bus numbers.
>   No one liked this but it worked :)
> 
> The other little wrinkle would be the need to define full bindings for CXL
> on DT + implement a fairly complex kernel stack as equivalent in ACPI
> involves a static table, CEDT, new runtime queries via _DSM and a description
> of various components. Doable, but so far there is no interest on physical
> platforms. Worth noting that for now, the QEMU CXL emulation is all about
> testing and developing the OS stack, not about virtualization (performance
> is terrible except in some very contrived situations!)
> 
> Back to posting as an RFC because there was some discussion of approach to
> modelling the devices that may need a bit of redesign.
> The discussion kind of died out on the back of DT issue and I doubt anyone
> can remember the details.
> 
> https://lore.kernel.org/qemu-devel/20220616141950.23374-1-Jonathan.Cameron@huawei.com/
> 
> There is only a very simple test in here, because my intent is not to
> duplicate what we have on x86, but just to do a smoke test that everything
> is hooked up.  In general we need much more comprehensive end to end CXL
> tests but that requires a reaonsably stable guest software stack. A few
> people have expressed interest in working on that, but we aren't there yet.
> 
> Note that this series has a very different use case to that in the proposed
> SBSA-ref support:
> https://lore.kernel.org/qemu-devel/20250117034343.26356-1-wangyuquan1236@phytium.com.cn/
> 
> SBSA-ref is a good choice if you want a relatively simple mostly fixed
> configuration.  That works well with the limited host system
> discoverability etc as EDK2 can be build against a known configuration.
> 
> My interest with this support in arm/virt is support host software stack
> development (we have a wide range of contributors, most of whom are working
> on emulation + the kernel support). I care about the weird corners. As such
> I need to be able to bring up variable numbers of host bridges, multiple CXL
> Fixed Memory Windows with varying characteristics (interleave etc), complex
> NUMA topologies with wierd performance characteristics etc. We can do that
> on x86 upstream today, or my gitlab tree. Note that we need arm support
> for some arch specific features in the near future (cache flushing).
> Doing kernel development with this need for flexibility on SBSA-ref is not
> currently practical. SBSA-ref CXL support is an excellent thing, just
> not much use to me for this work.
> 
> Jonathan Cameron (5):
>  hw/cxl-host: Add an index field to CXLFixedMemoryWindow
>  hw/cxl: Make the CXL fixed memory windows devices.
>  hw/cxl-host: Allow split of establishing memory address and mmio
>    setup.
>  hw/arm/virt: Basic CXL enablement on pci_expander_bridge instances
>    pxb-cxl
>  qtest/cxl: Add aarch64 virt test for CXL
> 
> include/hw/arm/virt.h     |   4 +
> include/hw/cxl/cxl.h      |   4 +
> include/hw/cxl/cxl_host.h |   6 +-
> hw/acpi/cxl.c             |  83 +++++++++------
> hw/arm/virt-acpi-build.c  |  34 ++++++
> hw/arm/virt.c             |  29 +++++
> hw/cxl/cxl-host-stubs.c   |   8 +-
> hw/cxl/cxl-host.c         | 218 ++++++++++++++++++++++++++++++++------
> hw/i386/pc.c              |  51 ++++-----
> tests/qtest/cxl-test.c    |  59 ++++++++---
> tests/qtest/meson.build   |   1 +
> 11 files changed, 389 insertions(+), 108 deletions(-)
> 
> -- 
> 2.43.0
> 

With your series applied on top of upstream QEMU, the -drive option does not work well with the sane CXL
setup (I use run_qemu.sh maintained by Marc et al. at Intel) see below:

/home/realm/projects/qemu/build/qemu-system-aarch64 -machine virt,accel=tcg,cxl=on,highmem=on,compact-highmem=on,highmem-ecam=on,highmem-mmio=on -m 2048M,slots=0,maxmem=6144M -smp 2,sockets=1,cores=2,threads=1 -display none -nographic -drive if=pflash,format=raw,unit=0,file=AAVMF_CODE.fd,readonly=on -drive if=pflash,format=raw,unit=1,file=AAVMF_VARS.fd -drive file=root.img,format=raw,media=disk -kernel mkosi.extra/boot/vmlinuz-6.15.0-rc4-00040-g128ad8fa385b -initrd mkosi.extra/boot/initramfs-6.15.0-rc4-00040-g128ad8fa385b.img -append selinux=0 audit=0 console=tty0 console=ttyS0 root=PARTUUID=14d6bae9-c917-435d-89ea-99af1fa4439a ignore_loglevel rw initcall_debug log_buf_len=20M memory_hotplug.memmap_on_memory=force cxl_acpi.dyndbg=+fplm cxl_pci.dyndbg=+fplm cxl_core.dyndbg=+fplm cxl_mem.dyndbg=+fplm cxl_pmem.dyndbg=+fplm cxl_port.dyndbg=+fplm cxl_region.dyndbg=+fplm cxl_test.dyndbg=+fplm cxl_mock.dyndbg=+fplm cxl_mock_mem.dyndbg=+fplm systemd.set_credential=agetty.autologin:root systemd.set_credential=login.noauth:yes -device e1000,netdev=net0,mac=52:54:00:12:34:56 -netdev user,id=net0,hostfwd=tcp::10022-:22 -cpu max -object memory-backend-file,id=cxl-mem0,share=on,mem-path=cxltest0.raw,size=256M -object memory-backend-file,id=cxl-mem1,share=on,mem-path=cxltest1.raw,size=256M -object memory-backend-file,id=cxl-mem2,share=on,mem-path=cxltest2.raw,size=256M -object memory-backend-file,id=cxl-mem3,share=on,mem-path=cxltest3.raw,size=256M -object memory-backend-file,id=cxl-lsa0,share=on,mem-path=lsa0.raw,size=128K -object memory-backend-file,id=cxl-lsa1,share=on,mem-path=lsa1.raw,size=128K -object memory-backend-file,id=cxl-lsa2,share=on,mem-path=lsa2.raw,size=128K -object memory-backend-file,id=cxl-lsa3,share=on,mem-path=lsa3.raw,size=128K -device pxb-cxl,id=cxl.0,bus=pcie.0,bus_nr=53 -device pxb-cxl,id=cxl.1,bus=pcie.0,bus_nr=191 -device cxl-rp,id=hb0rp0,bus=cxl.0,chassis=0,slot=0,port=0 -device cxl-rp,id=hb0rp1,bus=cxl.0,chassis=0,slot=1,port=1 -device cxl-rp,id=hb1rp0,bus=cxl.1,chassis=0,slot=2,port=0 -device cxl-rp,id=hb1rp1,bus=cxl.1,chassis=0,slot=3,port=1 -device cxl-upstream,port=4,bus=hb0rp0,id=cxl-up0,multifunction=on,addr=0.0,sn=12345678 -device cxl-switch-mailbox-cci,bus=hb0rp0,addr=0.1,target=cxl-up0 -device cxl-upstream,port=4,bus=hb1rp0,id=cxl-up1,multifunction=on,addr=0.0,sn=12341234 -device cxl-switch-mailbox-cci,bus=hb1rp0,addr=0.1,target=cxl-up1 -device cxl-downstream,port=0,bus=cxl-up0,id=swport0,chassis=0,slot=4 -device cxl-downstream,port=1,bus=cxl-up0,id=swport1,chassis=0,slot=5 -device cxl-downstream,port=2,bus=cxl-up0,id=swport2,chassis=0,slot=6 -device cxl-downstream,port=3,bus=cxl-up0,id=swport3,chassis=0,slot=7 -device cxl-downstream,port=0,bus=cxl-up1,id=swport4,chassis=0,slot=8 -device cxl-downstream,port=1,bus=cxl-up1,id=swport5,chassis=0,slot=9 -device cxl-downstream,port=2,bus=cxl-up1,id=swport6,chassis=0,slot=10 -device cxl-downstream,port=3,bus=cxl-up1,id=swport7,chassis=0,slot=11 -device cxl-type3,bus=swport0,persistent-memdev=cxl-mem0,id=cxl-pmem0,lsa=cxl-lsa0 -device cxl-type3,bus=swport2,persistent-memdev=cxl-mem1,id=cxl-pmem1,lsa=cxl-lsa1 -device cxl-type3,bus=swport4,volatile-memdev=cxl-mem2,id=cxl-vmem2,lsa=cxl-lsa2 -device cxl-type3,bus=swport6,volatile-memdev=cxl-mem3,id=cxl-vmem3,lsa=cxl-lsa3 -M cxl-fmw.0.targets.0=cxl.0,cxl-fmw.0.size=4G,cxl-fmw.0.interleave-granularity=8k,cxl-fmw.1.targets.0=cxl.0,cxl-fmw.1.targets.1=cxl.1,cxl-fmw.1.size=4G,cxl-fmw.1.interleave-granularity=8k -snapshot -object memory-backend-ram,id=mem0,size=2048M -numa node,nodeid=0,memdev=mem0, -numa cpu,node-id=0,socket-id=0 -numa dist,src=0,dst=0,val=10
qemu-system-aarch64: -drive file=root.img,format=raw,media=disk: PCI: Only PCI/PCIe bridges can be plugged into pxb-cxl

Plain upstream QEMU aarch64 target vert machine can handle the -drive option without an issue _without_ those cxl setup options added. I think the error was seen with your previous cxl-2025-03-20 branch. 

Thanks,
Itaru.