Re: [PATCH V10 4/5] arm64: apei: implement arch_apei_get_mem_attributes()

[Matt Fleming <matt@codeblueprint.co.uk>]

On Fri, 07 Aug, at 11:00:17AM, Ard Biesheuvel wrote:
> 
> The EFI memory types are not exclusive, and so many regions will have
> all of the above set. The UEFI spec does not define how to interpret
> these superimposed attributes, it is up to the OS to decide on a
> consistent approach.
> 
> For instance, this region (captured from a arm64 boot log with
> uefi_debug set on the command line)
> 
> [Runtime Data       |RUN|  |  |  |   |WB|WT|WC|UC]
> 
> would be mapped uncached when following the above logic, while it
> makes more sense to map using PAGE_KERNEL in this case.

Urgh... good point Ard. Right now this is limited to the GHES driver, so
it's unclear whether this patch is buggy in practice or not.

Does it *ever* make sense to map a region as cacheable (WB/WT/WC) on
arm64 for the APEI/GHES case? Does the firmware handle the necessary
cache flushing?

> From the spec:
> 
> """
> EFI_MEMORY_UC: The memory region supports being configured as not cacheable.
> EFI_MEMORY_WC: The memory region supports being configured as write combining.
> EFI_MEMORY_WT: The memory region supports being configured as
> cacheable with a “write through” policy. Writes that hit in the cache
> will also be written to main memory.
> EFI_MEMORY_WB: The memory region supports being configured as
> cacheable with a “write back” policy. Reads and writes that hit in the
> cache do not propagate to main memory. Dirty data is written back to
> main memory when a new cache line is allocated.
> """
 
Jonathan, can you please provide the EFI memory map region attributes
for the GHES region that requires this series?

-- 
Matt Fleming, Intel Open Source Technology Center