linux-kernel - Re: [patch 00/13] x86: PAT support updated

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Date:	Fri, 21 Mar 2008 12:59:48 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
CC:	ak@....de, ebiederm@...ssion.com, rdreier@...co.com,
	torvalds@...ux-foundation.org, gregkh@...e.de, airlied@...net.ie,
	davej@...hat.com, mingo@...e.hu, tglx@...utronix.de,
	akpm@...ux-foundation.org, arjan@...radead.org,
	jesse.barnes@...el.com, linux-kernel@...r.kernel.org,
	suresh.b.siddha@...el.com
Subject: Re: [patch 00/13] x86: PAT support updated - v3

Venki Pallipadi wrote:
> On Fri, Mar 21, 2008 at 06:29:48AM -0700, H. Peter Anvin wrote:
>> venkatesh.pallipadi@...el.com wrote:
>>> * UC_MINUS in PAT to provide backward compatibility to /dem/mem mmap users.
>> I have to say I think this looks like a good patchset.  However, I'd 
>> like a bit more clarification with regards to the above point?
>>
> 
> X seems to use (in that order)
> - mmap the range through /dev/mem
> - Set MTRR for the range to WC
> 
> I see this happening on one of my test systems with relatively new xorg.
> 
> In this case, when mmap does the reserve for this range, if we give UC mapping
> then we will effectively negate the MTRR WC setting with the range being mapped
> UC. To accomodate this special use case, we give /dev/mem mmap (only when there
> are no other already existing mappings) a UC_MINUS attribute. With that,
> if and when X sets MTRR the range will become WC and until that time it will be
> UC. We ensure that all page table mappings use UC_MINUS for that range.
> 
> Long term, we want X to switch to /proc/ of /sys interfaces. But, we can also
> provide backward compatibility for existing X usage like above.
> 

Makes total sense.  Eventually I think we want to do /proc/mtrr 
emulation, but for now, this is probably the best option.

	-hpa
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