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SubjectRe: [PATCH] [4/7] Don't use large pages to map the first 2/4MB of memory
On Wed, 12 Mar 2008, Andi Kleen wrote:
> Intel recommends to not use large pages for the first 1MB
> of the physical memory because there are fixed size MTRRs there
> which cause splitups in the TLBs.
>
> On AMD doing so is also a good idea.
>
> The implementation is a little different between 32bit and 64bit.
> On 32bit I just taught the initial page table set up about this
> because it was very simple to do. This also has the advantage
> that the risk of a prefetch ever seeing the page even
> if it only exists for a short time is minimized.
>
> On 64bit that is not quite possible, so use set_memory_4k() a little
> later (in check_bugs) instead.


> + /*
> + * Make sure the first 2MB area is not mapped by huge pages
> + * There are typically fixed size MTRRs in there and overlapping
> + * MTRRs into large pages causes slow downs.
> + *
> + * Right now we don't do that with gbpages because there seems
> + * very little benefit for that case.

And why exactly ? Does slowdown not matter with gbpages ?

Also we split the first GB mapping anyway due to the various regions
(NX, RO, UC) in there.

Thanks,

tglx


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