Re: [PATCH v2] x86/p2m-pt: tighten conditions of IOMMU mapping updates

[George Dunlap <george.dunlap@citrix.com>]

On 01/10/15 11:25, Jan Beulich wrote:
> Whether the MFN changes does not depend on the new entry being valid
> (but solely on the old one), and the need to update or TLB-flush also
> depends on permission changes.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Reviewed-by: George Dunlap <george.dunlap@citrix.com>

> ---
> v2: Split from larger patch. Fix logic determining whether to update/
>     flush IOMMU mappings.
> ---
> TBD: As already mentioned on the large-page-MMIO-mapping patch, there
>      is an apparent inconsistency with PoD handling: 2M mappings get
>      valid entries created, while 4k mappings don't. It would seem to
>      me that the 4k case needs changing, even if today this may only
>      be a latent bug. Question of course is why we don't rely on
>      p2m_type_to_flags() doing its job properly and instead special
>      case certain P2M types.

The inconsistency in the conditionals there is a bit strange; but I'm
pretty sure that in the 2MB case it is (at the moment) superfluous,
because at the moment it seems that when setting a page with type
p2m_populate_on_demand, it's always passing in _mfn(0), which is valid.

(It used to pass a magic MFN, but Tim Deegan switched it to _mfn(0) at
some point without comment.)