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

From: Ard Biesheuvel
Date: Fri Sep 04 2015 - 14:53:44 EST

On 4 September 2015 at 20:23, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 04 Sep, at 03:24:21PM, Ard Biesheuvel wrote:
>> Since the UEFI spec does not mandate an enumeration order for
>> GetMemoryMap(), it seems to me that you still need to sort its output
>> before laying out the VA space. Since you need to sort it anyway, why
>> not simply sort it in reverse order and keep all the original code?
>> Considering that this is meant for stable, that would keep the delta
>> *much* smaller.
> Hmm... that'd be a neat trick and while it would save on the diff
> size, I don't think it would be smaller in terms of change complexity.
> EDK2 sorts the memory map when EFI_PROPERTIES_TABLE is enabled, so we
> can be reasonably sure the entry order returned by GetMemoryMap() is
> compatible with the split regions, even if it's not mandated by the
> spec.

EDK2 does sort it, but the spec does not mandate it so another
implementation may do something different entirely.

> For the non-EFI_PROPERTIES_TABLE case, things have been working fine
> without the sorting, so I'm reluctant to introduce it now (it's also
> much less of an issue there).

I see. I do wonder, since the VA mapping preserves the modulo 2 MB
alignment of each region, aren't you using much more VA space when
mapping in reverse order as you are doing now?

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