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

[Heyi Guo <heyi.guo@linaro.org>]

OK, we'll make the change for D0x too.

Thanks,

Heyi

On 30 October 2017 at 23:17, Ard Biesheuvel <ard.biesheuvel@linaro.org>
wrote:

> On 30 October 2017 at 15:13, Heyi Guo <heyi.guo@linaro.org> wrote:
> > Hi Ard,
> >
> >
> > On 10/30/2017 04:21 PM, Ard Biesheuvel wrote:
> >>
> >> 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.
> >
> > I actually meant EFI_MOMORY_WC for I supposed EFI_MOMORY_WC should be
> enough
> > for AArch64 non cacheable DMA access.
> >
> >>
> >> 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 memoryEFI_MOMORY_WC by
> default,
> >> so you
> >> should be getting EFI_MEMORY_WC in most cases.
> >
> >
> > Oh, I supposed we always have UC capability for system memory and we
> > actually still do that on D0x platforms. Does it mean we'd better remove
> > this capability to get the issue fixed?
>
> Yes.
>
> > Is there any architectural reason
> > for not setting UC capability on system memory?
> >
>
> Yes, exactly the reasons you mention: memory no longer behaves as
> memory if you map it with EFI_MEMORY_UC attributes, i.e., unaligned
> accesses or DC ZVA instructions can no longer be used.
>