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

[Ingo Molnar <mingo@kernel.org>]

* Ard Biesheuvel <ard.biesheuvel@linaro.org> 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.

> [...] 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.

Thanks,

	Ingo