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Message-ID: <4947FC68.4020603@goop.org>
Date: Tue, 16 Dec 2008 11:07:20 -0800
From: Jeremy Fitzhardinge <jeremy@...p.org>
To: Ingo Molnar <mingo@...e.hu>
CC: "Rafael J. Wysocki" <rjw@...k.pl>,
Martin Steigerwald <ms@...mix.de>,
linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: physical memory limit of 64-bit linux
Ingo Molnar wrote:
> phase 3) We could also go close to 47 bits: with various more invasive
> movings of VMALLOC and rest upwards, and other considerations
> such as the elimination of the generous start of 8 TB hole at
> __PAGE_OFFSET - i.e. moving __PAGE_OFFSET straight down to
> minus 128 TB. 120 TB would be doable.
>
Originally it was there, but I moved it up because that's where Xen puts
itself when running a PV 64-bit guest. It is also properly
parameterised now, so we could make it move on the basis of a config
setting.
> phase 4) If the 48 bits limit is ever lifted on the CPU side, we can move
> __PAGE_OFFSET down. This is actually less invasive than phase
> 3), because moving __PAGE_OFFSET is relatively easy. The far
> more invasive change would be the necessary changes to the
> virtual memory code: the current 4-level paging has a 256 TB
> limit which comes from the 512*512*512*512*4K split of
> pgd/pud/pmd/pte entries. Either PGDIR_SHIFT would have to be
> increased, moving the root pgtable's size from 4K to 8K or more,
> or another pgdir level would have to be introduced (which is
> even more intrusive and much less likely to be implemented by hw
> makers).
>
...or we could just reintroduce highmem ;)
J
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