linux-kernel - Re: [PATCH] Fix devmem_is_allowed for below 1MB accesses for an efi machine

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Date:	Thu, 04 Oct 2012 14:06:09 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Randy Wright <rwright@...com>
CC:	Matthew Garrett <mjg@...hat.com>, linux-kernel@...r.kernel.org,
	tmac@...com
Subject: Re: [PATCH] Fix devmem_is_allowed for below 1MB accesses for an efi
 machine

On 10/04/2012 01:07 PM, Randy Wright wrote:
> 
> I wanted to mention in this context a patch RFC I posted yesterday as a
> distinct thread which is visible as https://lkml.org/lkml/2012/10/3/589
> with the subject: [PATCH RFC] function probe_roms accessing improper
> addresses on UEFI systems.
> 

And it's equally as wrong.  It has nothing to do with UEFI vs BIOS at
all; this is rather a change in the classic behavior on x86 to return -1
on an impossible read rather than #MC.

Normally I would say probe_roms() doesn't really make any sense in the
EFI context anyway, but I believe there are systems which actually need
to probe at least for the video ROM even when running under EFI (and I
think there are storage devices which have parameter blocks in their
ROMs with similar issues).

Excluding reserved regions in general is a non-option, because on a lot
of systems the ROMs that *do* need to be probed for are marked just
RESERVED.

One option would be to quirk it; obviously there is some piece of
hardware which does cause this #MC and hopefully we could use that to
detect that specific regions should be excluded; another option would be
to trap the #MC during ROM probing.

	-hpa
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