On Thu, Jul 03, 2025 at 11:41:05AM +0100, Jonathan Cameron wrote: > v17: Thanks to Eric for review > - Add a comment to the high memory map to reduce the chance of nasty > surprises in the future as similar to device_memory, the CXL > Fixed Memory Windows are of variable size as so can't be represented > by explicit entries in the map. > - Updated a couple of patch descriptions to provide answers to questions > Eric raised.
I also tested the cxl/next kernel; it boots fine with CXL devices. Tested-by: Itaru Kitayama <itaru.kitay...@fujitsu.com> > > v16: > - Mostly additional documentation and descriptive text in > patch titles. > - Update test to reflect changes to other tests. > - Update physmem_max in virt to include the CXL memory. > > Updated 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>. Will update shortly with this > series. > > 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!) > > 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-wangyuquan1...@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. > > Also, we are kicking off some work on DCD virtualization, particularly to > support inter-host shared memory being presented up into a VM. That > will need upstream support on arm64 as it is built on top of the existing > CXL emulation to avoid the need for a separate guest software stack. > > Note this is TCG only - it is possible to support limited use with KVM but > that needs additional patches not yet ready for upstream. The challenge > is interleave - and the solution is don't interleave if you want to run > with KVM. > > Jonathan Cameron (5): > hw/cxl-host: Add an index field to CXLFixedMemoryWindow > hw/cxl: Make the CXL fixed memory windows devices. > hw/arm/virt: Basic CXL enablement on pci_expander_bridge instances > pxb-cxl > docs/cxl: Add an arm/virt example. > qtest/cxl: Add aarch64 virt test for CXL > > docs/system/arm/virt.rst | 9 ++ > docs/system/devices/cxl.rst | 11 +++ > include/hw/arm/virt.h | 4 + > include/hw/cxl/cxl.h | 5 +- > include/hw/cxl/cxl_host.h | 5 +- > hw/acpi/cxl.c | 76 ++++++++-------- > hw/arm/virt-acpi-build.c | 34 +++++++ > hw/arm/virt.c | 30 +++++++ > hw/cxl/cxl-host-stubs.c | 7 +- > hw/cxl/cxl-host.c | 174 +++++++++++++++++++++++++++++------- > hw/i386/pc.c | 50 +++++------ > tests/qtest/cxl-test.c | 58 +++++++++--- > tests/qtest/meson.build | 1 + > 13 files changed, 354 insertions(+), 110 deletions(-) > > -- > 2.48.1 >
