Until now, testing using CXL has relied up always using two root ports below a host bridge, to work around a current assumption in the Linux kernel support that, in the single root port case, the implementation will use the allowed passthrough decoder implementation choice. If that choice is made all accesses are routed from the host bridge to the single root port that is present. Effectively we have a pass through decoder (it is called that in the kernel driver).
This patch series implements that functionality and makes it the default See patch 2 for a discussion of why I think we can make this change without backwards compatibility issues (basically if it didn't work before who are we breaking by making it work?) Whilst this limitation has been known since the initial QEMU patch postings / kernel CXL region support, Fan Ni Ran into it recently reminding me that we should solve it. https://lore.kernel.org/linux-cxl/20230113171044.GA24788@bgt-140510-bm03/ Tree with a large set of patches before this at: https://gitlab.com/jic23/qemu/-/tree/cxl-2023-01-20 I've done some basic testing, though I did hit what appears to be a kernel race on region bring up of existing region / namespace in a 1HB 2RP 2EP test case. That is proving hard to replicate consistently but doesn't seem to have anything to do with the emulation other than perhaps we are opening up a race by responding slowly to something. Jonathan Cameron (2): hw/pci: Add pcie_count_ds_port() and pcie_find_port_first() helpers hw/pxb-cxl: Support passthrough HDM Decoders unless overridden hw/cxl/cxl-host.c | 31 +++++++++++++-------- hw/pci-bridge/pci_expander_bridge.c | 43 +++++++++++++++++++++++++---- hw/pci/pcie_port.c | 38 +++++++++++++++++++++++++ include/hw/cxl/cxl.h | 1 + include/hw/cxl/cxl_component.h | 1 + include/hw/pci/pci_bridge.h | 1 + include/hw/pci/pcie_port.h | 2 ++ 7 files changed, 100 insertions(+), 17 deletions(-) -- 2.37.2