Re: Xen's use of PAT and PV guests

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 03/30/2010 10:56 AM, Ian Campbell wrote:
> I had a patch ages ago (which I have now lost) that caused the kernel to
> read back the PAT MSR after writing it and try and locate a suitable
> entry for each cache setting it was interested in (with fallbacks as
> appropriate) to use dynamically thereafter.
>
> This has the nice property that Linux could write what it really wanted
> to the PAT register but it would then read and use whatever it actually
> ended up with.
>

I started to implement something like that, but stopped and decided to 
do a much simpler hack.  I wanted to make sure that make_pte and pte_val 
are proper inverses of each other, so pte_val needs to do the reverse 
mapping from xen pte pat flags -> linux pte pat flags.  Given that the 
only difference between Xen and Linux is whether _PAGE_PWT means WT or 
WC, it is easy to do the mapping forward and back.

But hugetlbfs adds the complication that it ends up constructing ptes 
via the pte operations (which makes logical sense), but on x86 that's a 
mess because the meaning of _PAGE_PAT changes to _PAGE_PSE on level 
2/3/4 entries, and there's no way of knowing what level we're looking at.

At the moment I'm winging it by ignoring _PAGE_PAT in make_pte, and hope 
that nobody wants to do pte_val on a hugetlbfs pte...

Looks like the proper fix is to stop hugetlbfs from using mk_pte, and 
add a mk_huge_tlb instead, and have the x86 version use the pmd operations.

> I'm not sure that this scheme is at all upstreamable though.
>

I don't see why not; it would all be hidden away in the Xen code, and 
maintains the normal x86 illusion.   It's just a matter of hooking 
wrmsr, make_pte and pte_val, which we do already.

     J