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

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Message-Id: <200702061154.57621.ak@suse.de>
Date:	Tue, 6 Feb 2007 11:54:57 +0100
From:	Andi Kleen <ak@...e.de>
To:	"Jan Beulich" <jbeulich@...ell.com>
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

On Tuesday 06 February 2007 10:53, Jan Beulich wrote:
> >> 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.

You couldn't use the memory in any ways.

Anyways I give up -- the check is probably not needed, unless Andreas
comes up with a good reason.

> 
> >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.

They would need a discontig kernel to boot most likely, otherwise
mem_map would fill up their memory. 

And I was told Windows doesn't like that, so it's unlikely there will ever be such
x86 machines.

-Andi
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