linux-kernel - Re: [2.6.25-git18 => 2.6.26-rc1-git1] Xorg crash with xf86MapVidMem error

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Date:	Thu, 08 May 2008 17:49:48 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
CC:	Rufus & Azrael <rufus-azrael@...ericable.fr>,
	Ingo Molnar <mingo@...e.hu>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [2.6.25-git18 => 2.6.26-rc1-git1] Xorg crash with	xf86MapVidMem
 error

Venki Pallipadi wrote:
> 
> What I meant was:
> MTRRs are not really base and size. They are defined as base and mask.
> Any addr is affected by mtrr if addr & mask == base & mask.
> So, MTRR entry like
> base = 0xf00000, mask = 0xff00000 with 36 bit physical address covers
> 0xf00000-0xffffff, 0x10f00000-0x10ffffff, 0x20f00000-0x20ffffff, ....
> 
> In this case if user is trying to mmap 0x1a000000-0x2a000000, we cannot really
> cover this case with single parsing of variable address ranges. We will have
> to go through the sub-ranges withing single variable range, which can be page
> by page in worst case.
>  

In practice, though, such MTRRs are never seen.  Even in the presence of 
such pathological MTRRs, I'm sure one can figure out a *much* smarter 
overlap algorithm.  I'd have to sit down and think about it, but I'm 
pretty sure one could; the basic observation, though, is that any set 
mask bit that are in a position <= floor(log2(range_len))-1 don't matter 
at all, since the range WILL end up covering both the 0 and the 1 case 
in this bit position.  At this point, you can test only a limited number 
of points (I *believe* you can reduce it down to only the beginning and 
the end, but I haven't proven that, so don't count on it yet.)

The key, of course, is to look for the case of multiple MTRRs matching 
the range.

	-hpa
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