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xen-devel
Re: [Xen-devel] [Linux PATCH] Fix to hugepages to work around new PWT ha
On 06/09/2010 08:26 PM, Dave McCracken wrote:
>> But in general kernel code shouldn't be just nakedly setting present on
>> the pte without also remaking the whole thing. That doesn't happen with
>> normal ptes, and it probably shouldn't happen with huge ptes. Forcing
>> present on a pte at this level seems very bogus. Why not change the
>> upper code to set present if that's want it wants?
>>
>> I'll skip this chunk for now.
>>
> Um, this is the upper level code. The entire purpose of make_huge_pte is to
> construct a present huge pte from page and pgprot. The problem is that the
> original code makes the pte, then sets the present bit via pte_mkhuge().
> This
> means the Xen-specific macro that triggers on present is misled and doesn't
> do
> the pfn_to_mfn(). Without this patch hugepages is handing pfns to the
> hypervisor to map instead of mfns.
>
In principle, setting present should cause the pte to be converted from
pfn to mfn, but I don't think that ever happens with normal ptes (since
non-present ptes contain swap info). But I don't see where a huge pte
gets present set; pte_mkhuge itself doesn't do anything except set PSE.
J
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