Re: [PATCH v5 4/4] MdePkg/BaseMemoryLibOptDxe ARM|AARCH64: disallow use in SEC & PEI phases

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 5 April 2017 at 21:12, Jeremy Linton <jeremy.linton@arm.com> wrote:
> Hi,
>
> On 09/09/2016 09:00 AM, Ard Biesheuvel wrote:
>>
>> The new accelerated ARM and AARCH64 implementations take advantage of
>> features that are only available when the MMU and Dcache are on. So
>> restrict the use of this library to the DXE phase or later.
>
>
> I don't think this is sufficient because DC ZVA doesn't work against device
> memory/etc. That means that users have to somehow know the page/etc
> attributes of memory regions before they call SetMemXX() on them.
>

Yes. I literally found this out myself yesterday. Note that this
applies equally to unaligned accesses.


> I think this is a problem because nowhere in the UEFI specs do I see such
> restrictions on those memory operations.
>

Using device attributes for memory is something we should ban for
AArch64 in the spec.

> For a specific problematic example, the LcdGraphicsOutputBlt.c uses it for
> BltVideoFill() and the target of that is likely not regular cached video
> memory.
>

Those drivers should be using EFI_MEMORY_WC not EFI_MEMORY_UC for the
VRAM mapping. Note that EFI_MEMORY_UC is nGnRnE which is unnecessarily
restrictive.

I agree there is a general issue here which we should address by
tightening the spec. I don't see a lot of value in avoiding DC ZVA and
unaligned accesses altogether, I'd rather fix the code instead.

Thanks,
Ard.