On Thu, Mar 06, 2025 at 12:08:49PM -0500, Gregory Price wrote: > On Thu, Mar 06, 2025 at 09:37:49AM +0800, Yuquan Wang wrote: > > On Wed, Mar 05, 2025 at 05:20:52PM -0500, Gregory Price wrote: > > First, thank you for bringing this up, this is exactly the type of > ambiguiuty i was hoping others would contribute. It's difficult to > figure out if the ACPI tables are "Correct", if there's unimplemented > features, or we're doing something wrong - because some of this is > undocumented theory of operation. > Thank you for your patience in replying my questions. :) > > > ================== > > > NUMA node creation > > > =================== > > > NUMA nodes are *NOT* hot-pluggable. All *POSSIBLE* NUMA nodes are > > > identified at `__init` time, more specifically during `mm_init`. > > > > > > What this means is that the CEDT and SRAT must contain sufficient > > > `proximity domain` information for linux to identify how many NUMA > > > nodes are required (and what memory regions to associate with them). > > > > > Condition: > > 1) A UMA/NUMA system that SRAT is absence, but it keeps CEDT.CFMWS > > 2）Enable CONFIG_ACPI_NUMA > > > > Results: > > 1) acpi_numa_init: the fake_pxm will be 0 and send to acpi_parse_cfmws() > > 2）If dynamically create cxl ram region, the cxl memory would be assigned > > to node0 rather than a fake new node. > > > > This is very interesting. Can I ask a few questions: > > 1) is this real hardware or a VM? Qemu VM (arm64 virt). > 2) By `dynamic creation` you mean leveraging cxl-cli (ndctl)? Yes. After boot, I used "cxl create-region". > 2a) Is the BIOS programming decoders, or are you programming the > decoder after boot? Program the decoder after boot. It seems like currently bios for qemu could not programm cxl both on x86(q35) and arm64(virt). I am trying to find a cxl-enable bios for qemu virt to do some test. > > > > Confusions: > > 1) Does CXL memory usage require a numa system with SRAT? As you > > mentioned in SRAT section: > > > > "This table is technically optional, but for performance information > > to be enumerated by linux it must be present." > > > > Hence, as I understand it, it seems a bug in kernel. > > > > It's hard to say if this is a bug yet. It's either a bug, or your > system should have an SRAT to describe what the BIOS has done. > > > 2) If it is a bug, could we forbid this situation by adding fake_pxm > > check and returning error in acpi_numa_init()? > > > > > 3）If not, maybe we can add some kernel logic to allow create these fake > > nodes on a system without SRAT? > > > > I think we should at least provide a warning (if the SRAT is expected > but missing) - but lets get some more information first. > > ~Gregory
