Re: another new version of pageattr caching conflict fix for 2.4

[Andi Kleen]

> > MTRRs work on physical, not virtual memory, so they have no aliasing
> > issues.
> 
> Doesn't the AGP aperture cause a physical alias?  Leading to strange

Yes. That's what this patch is all about.

> the same problems if the agp aperture was marked write-back, and the

AGP aperture is uncacheable, not write-back.

> memory was marked uncacheable.  My gut impression is to just make the
> agp aperture write-back cacheable, and then we don't have to change
> the kernel page table at all.  Unfortunately I don't expect the host

That would violate the AGP specification.

> bridge with the memory and agp controllers to like that mode,
> especially as there are physical aliasing issues.

exactly.

> 
> > Fixing the MTRRs is fine, but it is really outside the scope of my patch.
> > Just changing the kernel map wouldn't be enough to fix wrong MTRRs,
> > because it wouldn't cover highmem. 
> 
> My preferred fix is to use PAT, to override the buggy mtrrs.  Which
> brings up the same aliasing issues.  Which makes it related but
> outside the scope of the problem.

I don't follow you here. IMHO it is much easier to fix the MTRRs in the
MTRR driver for those rare buggy BIOS (if they exist - I've never seen one)
than to hack up all of memory management just to get the right bits set.
I see no disadvantage of using the MTRRs and it is lot simpler than
PAT and pte bits.

-Andi