Re: [PATCH v4 6/6] x86/P2M: relax permissions of PVH Dom0's MMIO entries

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

On Wed, Sep 29, 2021 at 03:15:48PM +0200, Jan Beulich wrote:
> To become independent of the sequence of mapping operations, permit
> "access" to accumulate for Dom0, noting that there's not going to be an
> introspection agent for it which this might interfere with. While e.g.
> ideally only ROM regions would get mapped with X set, getting there is
> quite a bit of work. Plus the use of p2m_access_* here is abusive in the
> first place.

While doing this might be fine on Intel hardware, AMD hardware can
specify strict mapping access requirements from the IVMD flags, and
hence we should enforce those.

I think a better solution would be to not return error if the only
divergence between the current mapping and the requested one is the
access flag. We could log a message in that case about being unable to
change the access for the gfn.

This relies on the RMRR/IVMD regions being setup before any other MMIO
region, or else Xen would have to clear existing entries on that case.

Thanks, Roger.