Re: [PATCH for-4.19? 1/2] xen/x86: account for max guest gfn and number of foreign mappings in the p2m

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, May 06, 2024 at 12:07:33PM +0200, Jan Beulich wrote:
> On 30.04.2024 18:58, Roger Pau Monne wrote:
> > Keep track of the maximum gfn that has ever been populated into the p2m, and
> > also account for the number of foreign mappings.  Such information will be
> > needed in order to remove foreign mappings during teardown for HVM guests.
> 
> Is "needed" the right term? We could e.g. traverse the P2M tree (didn't look
> at patch 2 yet as to how exactly you use these two new fields there), at which
> point we might get away without either or both of these extra statistics,
> while at the same time also not needing to iterate over a gigantic range of
> GFNs. Going from populated page tables would roughly match "max_gfn", with the
> benefit of certain removals of P2M entries then also shrinking the upper 
> bound.

One note about traversing the p2m tree that I forgot to add earlier:
AFAICT we would need one implementation for EPT and one for NPT, as I
expect the different page-table format won't allow us to use the same
code against both EPT and NPT page-tables (I really need to check).

Thanks, Roger.