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xen-devel
Re: [Xen-devel] [Linux PATCH] Fix to hugepages to work around new PWT ha
On Wednesday, June 09, 2010, Jeremy Fitzhardinge wrote:
> >>> An additional fix here is to make sure the _PAGE_PRESENT bit is set
> >>> before hugepages does a mk_pte(), since Xen depends on that bit to
> >>> trigger the pfn->mfn translation.
> >>>
> >>>
> >>>
> >>>
> >> Why is the kernel creating a non-present mapping? If it isn't present,
> >> why does it matter whether we do the pfn->mfn conversion?
> >>
> >>
> >>
> > The hugepage function make_huge_pte() called mk_pte() to turn a page and
> > a pgprot into a pte before it set PRESENT. The PRESENT flag was set
> > after the pte was made. This meant that the Xen version of the macro
> > did not see PRESENT so did not do the pfn_to_mfn(). My patch sets
> > PRESENT first so the right thing will happen.
> >
> >
>
> 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.
Dave
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