From: "Jan Beulich" <jbeulich@novell.com>
To: "Jeremy Fitzhardinge" <jeremy@goop.org>
Cc: "Ingo Molnar" <mingo@elte.hu>, "Andi Kleen" <andi@firstfloor.org>,
"Stable Kernel" <stable@kernel.org>, <x86@kernel.org>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit
Date: Fri, 06 Jun 2008 09:14:04 +0100 [thread overview]
Message-ID: <48490DEC.76E4.0078.0@novell.com> (raw)
In-Reply-To: <4848EE5C.6040003@goop.org>
>>> Jeremy Fitzhardinge <jeremy@goop.org> 06.06.08 09:59 >>>
>Andi Kleen wrote:
>> Jeremy Fitzhardinge <jeremy@goop.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?
No, it's the fact that the 1:1 mapping needs as much virtual space as
the physical range covered (including all holes).
Jan
next prev parent reply other threads:[~2008-06-06 8:13 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-06-05 15:21 [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit Jeremy Fitzhardinge
2008-06-05 15:34 ` Jan Beulich
2008-06-05 15:42 ` Jeremy Fitzhardinge
2008-06-05 16:45 ` H. Peter Anvin
2008-06-05 21:14 ` Jeremy Fitzhardinge
2008-06-07 18:35 ` H. Peter Anvin
2008-06-06 9:21 ` [PATCH UPDATED] x86: set PAE PHYSICAL_MASK_SHIFT to 44 bits Jeremy Fitzhardinge
2008-06-06 9:58 ` Jan Beulich
2008-06-06 13:15 ` Andi Kleen
2008-06-06 13:50 ` Jeremy Fitzhardinge
2008-06-10 10:31 ` Ingo Molnar
2008-06-10 13:06 ` Jeremy Fitzhardinge
2008-06-13 7:24 ` Ingo Molnar
2008-06-06 1:40 ` [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit Andi Kleen
2008-06-06 7:14 ` Jan Beulich
2008-06-06 7:59 ` Jeremy Fitzhardinge
2008-06-06 8:14 ` Jan Beulich [this message]
2008-06-06 8:15 ` Jeremy Fitzhardinge
2008-06-07 18:39 ` H. Peter Anvin
2008-06-06 4:45 ` Arjan van de Ven
2008-06-06 8:08 ` Jeremy Fitzhardinge
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