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Date: Fri, 06 Jun 2008 08:59:24 +0100
From: Jeremy Fitzhardinge <jeremy@...p.org>
To: Andi Kleen <andi@...stfloor.org>
CC: Ingo Molnar <mingo@...e.hu>, x86@...nel.org,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Jan Beulich <jbeulich@...ell.com>,
Stable Kernel <stable@...nel.org>
Subject: Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit
Andi Kleen wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
>
>> The 46-bit mask used in 64-bit seems pretty arbitrary.
>>
>
> The rationale for the 46 bits is that the kernel needs roughly 4x as
> much virtual space as physical space and the virtual space is limited
> to 48bits.
>
> To be exact 47 bits is always user space and the 47 bits remaining
> for the kernel are split into half, with one half for the direct mapping
> and the other half for random mappings. With some pushing you could
> extend it to 46.5 bits or so, but beyond that you'll be in trouble.
>
Why's that? Is the issue the amount of memory needed for pagetables and
page structures if you did have more than 2^48 bytes of physical memory?
> It's not arbitrary at all.
I didn't say it was. That was the introduction to my explanation of why
I didn't think it was arbitrary. Of course, if there had been a comment
there explaining the rationale, I wouldn't have had to make one up...
J
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