On Thu, Mar 13, 2025 at 05:20:04PM +0000, Jonathan Cameron wrote: > Gregory Price <gourry@xxxxxxxxxx> wrote: > > > ------------------------------- > > One 2GB Device, Multiple CFMWS. > > ------------------------------- > > Lets imagine we have one 2GB device attached to a host bridge. > > > > In this example, the device hosts 2GB of persistent memory - but we > > might want the flexibility to map capacity as volatile or persistent. > > Fairly sure we block persistent in a volatile CFMWS in the kernel. > Any bios actually does this? > > You might have a variable partition device but I thought in kernel at > least we decided that no one was building that crazy? > This was an example I pulled from Dan's notes elsewhere (i think). I was unaware that we blocked mapping persistent as volatile. I was working off the assumption that could be flexible mapped similar to... er... older, non-cxl hardware... cough. > Maybe a QoS split is a better example to motivate one range, two places? > That probably makes sense? > > ------------------------------------------------------------- > > Two Devices On One Host Bridge - With and Without Interleave. > > ------------------------------------------------------------- > > What if we wanted some capacity on each endpoint hosted on its own NUMA > > node, and wanted to interleave a portion of each device capacity? > > If anyone hits the lock on commit (i.e. annoying BIOS) the ordering > checks on HPA kick in here and restrict flexibility a lot > (assuming I understand them correctly that is) > > This is a good illustration of why we should at some point revisit > multiple NUMA nodes per CFMWS. We have to burn SPA space just > to get nodes. From a spec point of view all that is needed here > is a single CFMWS. > Along with the above note, and as mentioned on discord, I think this whole section naturally evolves into a library of "Sane configurations" and "We promise nothing for `reasons`" configurations. Maybe that turns into a kernel doc section that requires updating if a platform disagrees / comes up with new sane configurations. This is certainly the most difficult area to lock down because we have no idea who is going to `innovate` and how. ~Gregory
