linux-kernel - Re: [PATCH] x86/PCI: never allocate PCI space from the last 1M below 4G

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Message-ID: <4CF41C89.9060708@zytor.com>
Date:	Mon, 29 Nov 2010 13:35:05 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Bjorn Helgaas <bjorn.helgaas@...com>
CC:	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Matthew Garrett <mjg@...hat.com>
Subject: Re: [PATCH] x86/PCI: never allocate PCI space from the last 1M below
 4G

On 11/29/2010 01:32 PM, Bjorn Helgaas wrote:
> 
> Oops, egg on my face.  In this case, there *is* an ACPI INT0800 device
> at 0xff000000-0xffffffff, which should prevent us from allocating that
> space for anything else.  Only problem is, we IGNORE that useful bit of
> information.
> 

Eep... why?

>> It is certainly reasonable to block off the last chunk of the 32-bit
>> address space.  Some systems double-decode it to avoid issues with
>> A20M#, so I would argue that we should avoid at least 2 MiB.
>>
>> As far as discovering them from the BIOS, there is a way to do it --
>> E820.  This is a fallback for the case where the BIOS has plain and
>> simply failed to provide it, and so a heuristic is probably the best we
>> can do.  Probing is extremely unsafe.
> 
> I think it's clearly a bug that Linux ignores ACPI resource information
> (except PNP0C01/PNP0C02 motherboard devices).  If we fix that bug, it
> will fix Matthew's 2530p.

I would definitely agree with that.

> We might still want a patch like this current one because it could
> work around some BIOS defects, and because I think it's too late to
> fix the ACPI resource problem for .37.  But I'm not convinced we
> should reserve more than Windows does, because that may keep us from
> discovering other important Linux problems.

I'm not so sure about that... it feels like a pretty weak argument that
we might work on some machines even though our code isn't perfect.

	-hpa
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