Re: [RFC] MdeModulePkg/NonDiscoverablePciDeviceDxe: NonCoherentPciIoAllocateBuffer issue with AArch64

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

On 30 October 2017 at 03:52, Heyi Guo <heyi.guo@linaro.org> wrote:
> Hi folks,
>
> In NonDiscoverablePciDeviceDxe driver, NonCoherentPciIoAllocateBuffer may
> allocate EFI_MEMORY_UC buffer depending on input Attributes and GCD
> capabilities. If it does, it actually allocates memory of "device" type in
> AArch64, but not "normal uncacheable" memory. For "device" memory type, it
> requires restrict access alignment and it may trigger alignment fault
> exception with BaseMemoryLibOptDxe in which read/write alignment is not
> guaranteed.
>
> Is EFI_MOMORY_WC enough for AArch64 platforms? How about other platforms,
> like X86?
>

Hello Heyi,

Do you mean EFI_MEMORY_UC in the last sentence? If not, I don't
understand the question.

Anyway, in reality, this code will only allocate EFI_MEMORY_UC memory
if any memory already exists in the memory map with that capability,
otherwise it will fall back to EFI_MEMORY_WC. On most arm64 platforms,
we no longer add this capability to system memory by default, so you
should be getting EFI_MEMORY_WC in most cases.

So the question is actually the opposite: does this interfere with
correct operation in cases where the shared mapping between the CPU
and the device should be strongly ordered, and EFI_MEMORY_WC doesn't
give sufficient guarantees.

-- 
Ard.