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.
> 
> Introduce a helper function to be used on both paths, such that
> respective checking differs in just the extra "to be unshared" condition
> on the fast path.
> 
> While there adjust the sanity check for foreign pages: Don't leak the
> reference on release builds when on a debug build the assertion would
> have triggered. (Thanks to Roger for the suggestion.)
> 
> Fixes: 80ea7af17269 ("x86/mm: Introduce get_page_from_gfn()")
> Fixes: 50fe6e737059 ("pvh dom0: add and remove foreign pages")
> Fixes: cbbca7be4aaa ("x86/p2m: make p2m_get_page_from_gfn() handle grant case 
> correctly")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.