linux-kernel - Re: [discuss] [patch] mtrr: fix issues with large addresses

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Date:	Tue, 06 Feb 2007 09:53:53 +0000
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Andi Kleen" <ak@...e.de>
Cc:	"Andreas Herrmann" <andreas.herrmann3@....com>,
	"Suresh B Siddha" <suresh.b.siddha@...el.com>,
	"Richard Gooch" <rgooch@...e-mbox.com>,
	<linux-kernel@...r.kernel.org>, <discuss@...-64.org>
Subject: Re: [discuss] [patch] mtrr: fix issues with large addresses

>> I don't think I remember a restriction here, at least not below 44 bits
>> (that's where pfn-s would need to become 64-bit wide).
>
>The i386 mm code only supports 4 entries in the PGD, so more than 36bit cannot 
>be mapped right now.

That has nothing to do with the number of physical address bits.

>Also even 64MB barely works (many boxes don't boot), you would likely
>need at least the 4:4 patch to go >64GB. Also we know there are tons
>of possible deadlocks in various subsystems when the lowmem:highmem ratio 
>gets so out of hand.
>
>Ok it could be probably all fixed with some work (at least the mm part,
>the deadlocks would be more tricky), but would seem fairly 
>pointless to me because all machines with >36bits support are 64bit capable.

That's a different story, and certainly a limiting factor. But this shouldn't
e.g. disallow (hypothetical?) systems that have a very sparse memory map
extending beyond 64G.

Jan
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