Re: [edk2-devel] [PATCH v3 1/1] MdeModulePkg: Add the EFI_RESOURCE_ATTRIBUTE_SPECIAL_PURPOSE attribute

["Oliver Smith-Denny" <osde@linux.microsoft.com>]

On 5/21/2024 8:40 PM, Du Lin wrote:
> Coherent Device Attribute Table (CDAT) specification defines a EFI memory type and attribute "EfiConventionalMemory Type with EFI_MEMORY_SP Attribute".
> Can we still support this type if assigning the GCD memory type "EfiGcdMemoryTypeReserved" for resource HOBs with the SPECIAL_PURPOSE attribute set?
> 
> CDAT 1.04 specification: https://uefi.org/sites/default/files/resources/Coherent%20Device%20Attribute%20Table_1.04%20published_0.pdf

Thanks for pointing out the CDAT specification. I think one of the
issues here is that the PI and UEFI specs are very vague on
EFI_MEMORY_SP, to the point that there is no user story on it.
This CDAT spec helps close some of the gap, but I think the
PI and UEFI spec should be updated to be more verbose in the
intended usage of this flag.

Does any CDAT code exist in edk2? A naive search didn't turn up
anything (and even looking at some of the CXL structures I didn't
see any usage outside of the header). Is the intention that the
CDAT would be read in PEI and a resource descriptor HOB created
from it? If so, I agree this would prevent EfiConventionalMemory
from being set. I think the CDAT spec should not be referencing
EFI memory types if that is the case, because resource descriptor
HOBs deal with resource types, not EFI types, so there is a mismatch.

If the CDAT is read at a different time and a resource descriptor
HOB is not created from it, then this should not affect it.

The reason that GCD reserved type was chosen here was that the
assumption for CXL or other remote memory is that UEFI would not
want to use it (what if the bus went down and DXE core was
allocated there). By marking it reserved with the EFI_MEMORY_SP
bit set, we ensure that UEFI does not use it and that the OS knows
that this is usable, but the kernel itself may not want to use it,
for the same reason (or for performance). Again, the UEFI spec does
not describe what memory type this should be, so if there is a different
use case here, please do share.

Thanks,
Oliver


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