Re: Bug report: the extended PCI config space is missed with 6.2-rc2

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Jan 05, 2023 at 01:43:20PM -0800, Dan Williams wrote:
> Bjorn Helgaas wrote:
> > On Thu, Jan 05, 2023 at 01:20:36PM -0800, Dan Williams wrote:
> > > Bjorn Helgaas wrote:
> > > > On Thu, Jan 05, 2023 at 11:44:28AM -0800, Dan Williams wrote:

> > > > > Would it be enough to add this clarification in "EFI 2.9 Table 7-6
> > > > > Memory Type Usage after ExitBootServices()"?
> > > > > 
> > > > > s/This memory is not used by the OS./This memory is not used by the OS,
> > > > > unless ACPI declares it for another purpose./
> > > > 
> > > > I guess the idea is that MCFG is a form of "ACPI declaring it"?  I
> > > > don't have an explicit citation for it, but I infer at [1] that ACPI
> > > > static tables are second-class citizens and not intended as a way of
> > > > reserving address space because that would lead to problems booting
> > > > old OSes on firmware that provides new tables unknown to the OS.
> > > 
> > > Ah, true, certainly for new stuff, but what about MCFG specifically?
> > > What harm is there an assuming that MMCONFIG intersecting with
> > > EfiMemoryMappedIO shall be treated as reserved for MMCONFIG usage.
> > 
> > Probably none, and I think that's what we'll have to do.  Ugh.
> > Another random special-case rule.
> > 
> > > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/PCI/acpi-info.rst?id=v6.1#n32
> 
> I am still holding out that a BIOS developer can either say "whoops,
> populating MMCONFIG in _CRS was overlooked", or point out "if you take
> the derivative of the PCI spec, multiply it be the inverse of the EFI
> spec and then take the cross-product with the ACPI spec then the memory
> type comes out as implicitly reserved".

Hahaha :)  Yep, but even if they change it, apparently there are lots
of machines in the field that won't get updated, so we're stuck
working around it.

Or, I guess the best-case scenario would be that it's not actually a
firmware bug, and there's some clean fix we can make to Linux.  But
I'm not holding my breath.