[PATCH 2/3] arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC

[Ard Biesheuvel]

On Thu, 6 May 2021 at 13:00, Mark Rutland <mark.rutland at arm.com> wrote:
>
> On Thu, May 06, 2021 at 11:46:41AM +0100, Will Deacon wrote:
> > On Thu, May 06, 2021 at 11:20:45AM +0100, Mark Rutland wrote:
> > > On Thu, May 06, 2021 at 10:50:33AM +0100, Will Deacon wrote:
> > > > The only user we have of Normal Write-Through memory is in the ACPI code
> > > > when mapping memory regions advertised as EFI_MEMORY_WT. Since most (all?)
> > > > CPUs treat write-through as non-cacheable under the hood, don't bother
> > > > with the extra memory type here and just treat EFI_MEMORY_WT the same way
> > > > as EFI_MEMORY_WC by mapping it to the Normal-NC memory type instead.
> > >
> > > The UEFI spec explicitly defines EFI_MEMORY_WT as Normal Outer-WT
> > > Inner-WT (and even explicitly specifies the MAIR.Attr<n> value).
> >
> > I wonder if they just did that because the names match :(
> >
> > > In the UEFI 2.9 spec, see section 2.3.6.1 "Memory types", Table 2-5
> > > "Map: EFI Cacheability Attributes to AArch64 Memory Types".
> > >
> > > The UEFI 2.9 spec can be found at:
> > >
> > >   https://uefi.org/sites/default/files/resources/UEFI_Spec_2_9_2021_03_18.pdf
> > >
> > > Given that is specified explicitly, and given that we don't know how
> > > future CPUs will treat this equivalently, I don't think this change is
> > > architecturally sound and I don't think there's wiggle-room to read the
> > > spec as permitting this.
> >
> > At the same time, allocating a MAIR for this memory type just because the
> > UEFI spec permits some theoretical future firmware to use it on some
> > theoretical CPU design is pretty farcical in my opinion. Looking through
> > current TRMs I've not been able to find a CPU that doesn't just emit
> > Normal-NC for this memory type.
> >
> > How about I add a pr_warn() in this case, so that we can revisit it in the
> > unlikely event that it ever comes up as an issue?
>
> If we also could avoid mapping EFI_MEMORY_WT to anything, that'd be
> nicer, but just the warning is probably good enough, yes.
>

EFI memory types are not exclusive. Most implementations use WB|WT|WC
for all memory, and some even use WB|WT|WC|UC, which means that the OS
can use whichever attributes it wants.

So we should only warn in cases where WT is specified but WC is not,
and I've never seen firmware that does this.