Re: [PATCH 1/2] x86/efi: Map EFI memmap entries in-order at runtime

[Matt Fleming]

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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