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

[Ard Biesheuvel]

On 29 September 2015 at 11:12, Ingo Molnar <mingo@xxxxxxxxxx> 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.
>
>> [...] 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.
>

OK, fair enough. I agree that setting the flag for 32-bit would be
semantically correct. I will leave it to Matt to comment whether it is
reasonable in terms of changes to other parts of the code.

Thanks,
Ard.
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