Re: [PATCH 2/4 v2] cxl/core: Add helpers to detect Low memory Holes on x86

[Gregory Price <gourry@gourry.net>]

On Tue, Apr 01, 2025 at 04:34:44PM -0700, Dan Williams wrote:
> The platforms with this condition want to support CXL mapped starting at
> zero *and* the typical/historical PCI MMIO space in low memory (for
> those PCI devices that do not support 64-bit addressing). If the CFMWS
> blindly followed the 256MB*NIW constraint the CXL window would overlap
> the MMIO space. So the choices are:
> 

If I'm understanding everything correctly, then I think this is intended
to work only when EFI_MEMORY_SP is *not* set for these particular CXL
devices - so it comes up as memory in early boot and we're just trying
to wire up all the bits to let the driver manage it accordingly?

If that's the case, then ignore me, i'm just bikeshedding at this point.

I can't imagine a system where the memory at 0x0-4GB is intended to come
up as EFI_MEMORY_SP, so I hope no one ever tries this :D

> > (Also, I still don't understand the oracle value of <4GB address range.
> >  It seems like if this is some quirk of SPA vs HPA alignment, then it
> >  can hold for *all* ocurrances, not just stuff below 4GB)
> 
> The goal is to get platform vendors to define the rules so that an OS
> has a reasonable expectation to know what is a valid vs invalid
> configuration. A hole above 4GB has no reason to exist, there is no
> resource conflict like PCI MMIO that explains why typical spec
> expectation can not be met.
> 
> So I want the subsystem to have an explicit set of platform quirks
> ideally backed up by updated spec language. That allows us to validate
> that the Linux implementation is correct by some objective source of
> truth, encourage platform vendors to update that source of truth when
> they create new corner cases, or even better, be more mindful to not
> create new corner cases.

I follow, seems reasonable.  

~Gregory