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

[Ard Biesheuvel]

On 7 August 2015 at 11:37, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
> 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?
>

No it does not. Currently, we only consider EFI_MEMORY_WB when
discovering system RAM from the UEFI memory map, so the direct linear
mapping should have a hole where the APEI/GHES regions lives if it
doesn't have the WB attribute set. This means we can map it WT/WC/UC
without violating architectural rules regarding mismatches attributes,
but it requires an explicit ioremap()

>> 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 âwrite throughâ 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 âwrite backâ 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
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