Messages in this thread | | | Date | Fri, 06 Jun 2008 09:14:04 +0100 | From | "Jan Beulich" <> | Subject | Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit |
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>>> 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
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