Re: [PATCH v2] x86/P2M: synchronize fast and slow paths of p2m_get_page_from_gfn()

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

On Tue, Mar 25, 2025 at 08:17:04AM +0100, Jan Beulich wrote:
> Handling of both grants and foreign pages was different between the two
> paths.
> 
> While permitting access to grants would be desirable, doing so would
> require more involved handling; undo that for the time being. In
> particular the page reference obtained would prevent the owning domain
> from changing e.g. the page's type (after the grantee has released the
> last reference of the grant). Instead perhaps another reference on the
> grant would need obtaining. Which in turn would require determining
> which grant that was.
> 
> Foreign pages in any event need permitting on both paths.

I've been wondering about this, and AFAICT the PV equivalent of this
function (the tail of get_page_from_gfn()) is much more limited in
that it just allows access to domain owned RAM or MMIO that has a
backing page (I expect mostly for the low 1MB?).

However for HVM domains we allow to take references to foreign mapped
pages and taking references to MMIO pages is not permitted.

Should the tail of get_page_from_gfn() also be adjusted to make use of
the newly introduced get_page_from_mfn_and_type(), thus unifying the
logic for both PV and HVM?

Could possibly be done in a separate change, I'm just trying to
understand why we have this seemingly different handling for PV vs
HVM.

Thanks, Roger.