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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.03.2025 10:00, Roger Pau Monné wrote:
> 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?

There's no (proper) use of P2M types in PV, so I don't think using this
function is viable for PV. In particular we'd never observe p2m_foreign
in PV, if I'm not mistaken.

> 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.

The difference is because access to foreign pages was hacked in to work
for PVH in a much different (and imo yet more hacky) way than it was
made work far longer ago for PV. The crucial part of that is in
get_page_from_l1e(), so get_page_from_gfn() isn't that relevant there
(I think).

Jan