Re: [patch 00/13] x86: PAT support updated - v3

["H. Peter Anvin" <hpa@zytor.com>]

Venki Pallipadi wrote:
> On Fri, Mar 21, 2008 at 06:29:48AM -0700, H. Peter Anvin wrote:
>> venkatesh.pallipadi@intel.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