linux-kernel - Re: [PATCH 0/4] EFI 1:1 mapping

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Date:	Mon, 03 Jun 2013 11:05:03 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Matthew Garrett <mjg59@...f.ucam.org>
Cc:	Borislav Petkov <bp@...en8.de>,
	Linux EFI <linux-efi@...r.kernel.org>,
	Matt Fleming <matt@...sole-pimps.org>,
	Jiri Kosina <jkosina@...e.cz>, X86-ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 0/4] EFI 1:1 mapping

On Mon, 2013-06-03 at 17:42 +0100, Matthew Garrett wrote:
> On Mon, Jun 03, 2013 at 09:35:07AM -0700, James Bottomley wrote:
> > On Mon, 2013-06-03 at 17:24 +0100, Matthew Garrett wrote:
> > > That seems optimistic. Windows never calls QueryVariableInfo() during 
> > > boot services, so what makes you think doing so has ever been tested?
> > 
> > It's used by the UEFI shell package ... every system which boots to the
> > shell automatically tests this.  I know no locked down UEFI system ships
> > with a shell but almost every system in test has a Shell in some form,
> > so I think its fairly safe to call it from boot services.
> 
> Why do you persist in this belief that all system vendors are going to 
> have run a shell, let alone any kind of test suite? That runs counter to 
> everything we've learned about x86 firmware. People verify that it runs 
> Windows and then ship it.

I don't, but I find it hard to believe no vendor ever runs an EFI shell
on their systems.  The feedback I got from a couple of OEMs is that they
use the shell mostly for internal testing.

> > However, what about a compromise: why don't we implement 1:1 mapping and
> > then call SetVirtualAddressMap with the 1:1 map ... in theory the
> > pointer chases should then be nops (it will be replacing the physical
> > address with the same virtual address), so everything should just work
> > and anything the UEFI vendor missed will still work because the physical
> > address will work also in this scenario.
> 
> The problem there is that you're saying "In theory". We know that 
> Windows doesn't behave this way, so we have no legitimate expectation 
> that it'll work. We know that it doesn't on some Apple hardware.

Fine, you say we need to call SetVirtualAddressMap because windows does,
I agree, I'm just saying we get additional safety from calling it with
the 1:1 map ... I don't see what the problem is.

James



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