On Tue, 29 Sep, at 11:12:30AM, Ingo Molnar wrote:
>
> * Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>
> > > except that I don't think
> > > the condition on 64-bit makes any sense:
> > >
> > > + if (!efi_enabled(EFI_OLD_MEMMAP) && efi_enabled(EFI_64BIT)) {
> > >
> > > I can see us being nervous wrt. backported patches, but is there any strong reason
> > > to not follow this up with a third (non-backported) patch that changes this to:
> > >
> > > + if (!efi_enabled(EFI_OLD_MEMMAP)) {
> > >
> > > for v4.4?
> > >
> >
> > The 32-bit side essentially implements the old memmap only, which is the the
> > bottom-up version. So old memmap will be implied by 32-bit but not set in the
> > EFI flags, resulting in the reverse enumeration being used with the bottom-up
> > mapping logic. The net result of that is that we create the same problem for
> > 32-bit that we are trying to solve for 64-bit, i.e., the regions will end up in
> > reverse order in the VA mapping.
> >
> > To deobfuscate this particular conditional, we could set EFI_OLD_MEMMAP
> > unconditionally on 32-bit x86. Or we could reshuffle variables and conditionals
> > in various other way.
>
> Setting EFI_OLD_MEMMAP would be fine, if doing that has no bad side effects.
Right, I think that's a very good suggestion, because like Ard
mentioned, since EFI_OLD_MEMMAP is implied for 32-bit (there's no
other way to map stuff currently), so it makes sense to force set the
bit.
> > [...] I am not convinced that the overall end result will be any better though.
>
> That's not true, we change an obscure, implicit dependency on 32-bit detail to an
> explicit EFI_OLD_MEMMAP flag that shows exactly what's happening. That's a clear
> improvement.
Agreed.
--
Matt Fleming, Intel Open Source Technology Center
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