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

From: Ingo Molnar <mingo@kernel.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Matt Fleming <matt.fleming@intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>,
	"Lee, Chun-Yi" <jlee@suse.com>, Borislav Petkov <bp@suse.de>,
	Leif Lindholm <leif.lindholm@linaro.org>,
	Peter Jones <pjones@redhat.com>,
	James Bottomley <JBottomley@odin.com>,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	Dave Young <dyoung@redhat.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Brian Gerst <brgerst@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/2] x86/efi: Map EFI memmap entries in-order at runtime
Date: Tue, 29 Sep 2015 11:12:30 +0200	[thread overview]
Message-ID: <20150929091230.GA2023@gmail.com> (raw)
In-Reply-To: <CAKv+Gu-e5CRrJ1S7phXyWCUZ10ptukn0zr0wDGyX14H_SpEp1Q@mail.gmail.com>

* Ard Biesheuvel <ard.biesheuvel@linaro.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.

> [...] 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.

Thanks,

	Ingo