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

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Message-ID: <20150929135256.GA4401@codeblueprint.co.uk>
Date:	Tue, 29 Sep 2015 14:52:56 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Matt Fleming <matt.fleming@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"Lee, Chun-Yi" <jlee@...e.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>,
	Dave Young <dyoung@...hat.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] x86/efi: Map EFI memmap entries in-order at runtime

On Tue, 29 Sep, at 11:12:30AM, Ingo Molnar wrote:
> 
> * Ard Biesheuvel <ard.biesheuvel@...aro.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.
 
Right, I think that's a very good suggestion, because like Ard
mentioned, since EFI_OLD_MEMMAP is implied for 32-bit (there's no
other way to map stuff currently), so it makes sense to force set the
bit.

> > [...] 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.

Agreed.

-- 
Matt Fleming, Intel Open Source Technology Center
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