Re: [PATCH 3/3] x86/P2M: don't include MMIO_DM in p2m_is_valid()

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Feb 26, 2025 at 12:53:14PM +0100, Jan Beulich wrote:
> MMIO_DM specifically marks pages which aren't valid, much like INVALID
> does. Dropping the type from the predicate
> - (conceptually) corrects _sh_propagate(), where the comment says that
>   "something valid" is needed (the only call path not passing in RAM_RW
>   would pass in INVALID_GFN along with MMIO_DM),
> - is benign to the use in sh_page_fault(), where the subsequent
>   mfn_valid() check would otherwise cause the same bail-out code path to
>   be taken,
> - is benign to all three uses in p2m_pt_get_entry(), as MMIO_DM entries
>   will only ever yield non-present entries, which are being checked for
>   earlier,
> - is benign to sh_unshadow_for_p2m_change(), for the same reason,
> - is benign to gnttab_transfer() with EPT not in use, again because
>   MMIO_DM entries will only ever yield non-present entries, and
>   INVALID_MFN is returned for those anyway by p2m_pt_get_entry().
> - for gnttab_transfer() with EPT in use (conceptually) corrects the
>   corner case of a page first being subject to XEN_DMOP_set_mem_type
>   converting a RAM type to MMIO_DM (which retains the MFN in the entry),
>   and then being subject to GNTTABOP_transfer, except that steal_page()
>   would later make the operation fail unconditionally anyway.
> 
> While there also drop the unused (and otherwise now redundant)
> p2m_has_emt().
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

It's tightening an existing check (making it more restrictive), so as
long as current users can deal with it.

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks, Roger.