Messages in this thread |  | | | Date | Fri, 06 Jun 2008 09:15:47 +0100 | | From | Jeremy Fitzhardinge <> | | Subject | Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit |
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Jan Beulich wrote: >>>> 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).
Right, I see. And suddenly 64-bits seems... constrained. ;)
J
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