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

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Message-ID: <CAKv+Gu-AQdgJNOtrgtEtqxsaSC_L=EANpC3pT8b6X2jmZ5qO_Q@mail.gmail.com>
Date:	Fri, 4 Sep 2015 20:53:36 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Borislav Petkov <bp@...e.de>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Peter Jones <pjones@...hat.com>,
	James Bottomley <JBottomley@...n.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Young <dyoung@...hat.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/efi: Map EFI memmap entries in-order at runtime

On 4 September 2015 at 20:23, Matt Fleming <matt@...eblueprint.co.uk> 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?

-- 
Ard.
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