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Re: [Xen-devel] Swapping of PT pages

At 02:14 +0530 on 20 Feb (1203473672), pradeep singh rautela wrote:
> I suspect Xen translates the mfns of the PV guests PT pages to
> equivalent pfns before they can be actually swapped out on to a
> physical disk.

Xen is not involved in the guest's swapping/paging activities.  If a
guest were to page its pagetables out to disk and then be migrated, they
would indeed be incorrect when they were reloaded.  The canonicalisation
happens only for pages that are currently in memory and marked with a
page-table type.

> > decided; if swapping of PT pages were allowed, then wouldn't certain
> > page table pages not be canonicalized (since they would be on disk,
> > not in any of the machine frames).  This obviously be bad.
> >
> > Am I understanding things incorrectly?

You're right.  If a guest wanted to use a pagetable after it had been
paged out, it would have to do the same MFN->PFN canonicalization on the
on-disk form, and PFN->MFN when reloading.

(That's PV guests, BTW.  HVM guests that swapped their pagetables could
potentially experience memory corruption if they used the emulated disk
controller, but should work fine with PV drivers, or with NPT.)


Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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