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

[Ard Biesheuvel]

On 8 September 2015 at 15:16, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 04 Sep, at 08:53:36PM, Ard Biesheuvel wrote:
>> 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.
>
> Yeah, we should get that requirement added to the spec.
>
>> > 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?
>
> It doesn't enforce a 2MB alignment for every entry, just those that
> are actually 2MB aligned. This should be exactly what was done in the
> previous version of the code. Do you see a bug?
>

I noticed that the 64-bit version of efi_map_region() preserves the
relative alignment with respect to a 2 MB boundary for /each/ region.
Since the regions are mapped in reverse order, it is highly unlikely
that each region starts at the same 2 MB relative alignment that the
previous region ended at, so you are likely wasting quite a bit of VA
space.

I don't think it is a bug, though, but it does not seem intentional.

-- 
Ard.
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