Hi Jonathan, On Wed, May 28, 2025 at 12:07:21PM +0100, Jonathan Cameron wrote: > v14: Simplifications suggeseted by Itaru (and some extra simplifications > that became apparent) and gather tags. > See individual patches for more information.
I think the suggestion was made by Zhi jian or Fan? who enaged in the rewview of your proposed series v13. Itaru. > > Updated cover letter > > Richard Henderson has posted a pull request with a fix for the TCG TLB > issue which will hopefully merge shortly (Thanks Richard!). > > 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/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 | 76 +++++++-------- > hw/arm/virt-acpi-build.c | 34 +++++++ > hw/arm/virt.c | 29 ++++++ > hw/cxl/cxl-host-stubs.c | 8 +- > hw/cxl/cxl-host.c | 190 ++++++++++++++++++++++++++++++++------ > hw/i386/pc.c | 51 +++++----- > tests/qtest/cxl-test.c | 59 +++++++++--- > tests/qtest/meson.build | 1 + > 11 files changed, 353 insertions(+), 109 deletions(-) > > -- > 2.48.1 > >
