Re: [discuss] [patch] mtrr: fix issues with large addresses

Eric W. Biederman
2007-02-06T19:45:45Z

"Joerg Roedel" <joerg.roedel@amd.com> writes:

> On Tue, Feb 06, 2007 at 12:08:12PM -0700, ebiederm@xmission.com wrote:
>> "Andreas Herrmann" <andreas.herrmann3@amd.com> writes:
>> > You are referring to current Linux implementation?
>> > The AMD64 architecture increased physical address size in PSE mode to
>> > 40 bits. So at least it would be possible to use more than 32 bits.
>>
>> How do you get 40 physical bits in a 32bit page table entry? My memory
>> is that the low bits in the page table entry were well defined and
>> accounted for. I'm pretty certain I can account for 6 of the low bits
>> off the top of my head. PSE is the page size extension allowing pages 2MB/4MB
>> pages.
>
> The access to 40 physical address bits is only possible using large pages
> (4MB on 32bit without PAE). In those page tables entrys you only use
> bits 22:31 for encoding the physical address. The bits 12:21 are
> unused. These unused bits are reused to encode bits 32:39 of the 40 bit
> physical address.

Yep. I missed that feature, and I do see it in AMD documentation now
that I look.

I'm not certain what that would be useful for though.

I'm pretty certain doesn't use this feature, we just enable PAE mode.

Eric