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

[Leif Lindholm <leif.lindholm@linaro.org>]

On Thu, Apr 06, 2017 at 10:43:57AM +0100, Ard Biesheuvel wrote:
> On 6 April 2017 at 10:35, Leif Lindholm <leif.lindholm@linaro.org> wrote:
> > On Wed, Apr 05, 2017 at 10:55:49PM +0100, Ard Biesheuvel wrote:
> >> >>> 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.
> >
> > Yes, completely agree. And doing so is generally the result of
> > misinderstanding the memory model (i.e., it probably won't provide the
> > guarantee that was sought).
> > Charles/Dong? Something to add to list?
> 
> As an additional note, the UEFI spec mandates that unaligned accesses
> are enabled for AArch64, which clearly expresses the intent that
> routines operating on memory should be able to do so without going out
> of its way to avoid unaligned accesses.

It does - but only if you already understand the memory model.

> > Can we insert a test preventing device memory type to be set for
> > regions with _WB attribute? Or is that already only possible through
> > manual trickery?
> 
> We should simply remove the _UC attribute from all memory. I have
> already done so for many of the platforms I more or less maintain (and
> for virt, we removed _WT and _WC as well, because KVM only supports
> _WB)

Agreed.

> Note that this does not prevent the NOR and RTC drivers from creating
> _UC regions for their own MMIO registers, it just prevents them from
> being remapped _UC via the DXE services.

OK, good.

/
    Leif