Re: [Xen-devel] [PATCH RFC 9/9] mm: mark pages that have their permissions controlled by a domain

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.07.14 at 15:34, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> This goes together with the mem-event API changes and marks certain
> pages as being controlled from outside the HV (a user domain in our
> case). This prevents Xen from resetting the permissions in certain
> cases, enforcing the previously expressed intention of receiving a
> memory event everytime the owning domain triggers a fault.

These "certain cases" would clearly benefit from being named in a
more precise fashion - it's not been that long ago that for some of
the cases where the access permissions got reset as a side effect
we decided to remove that side effect, and it was discussed to also
be done for at least one more case. I.e. perhaps you would not
need extensive changes like the ones here if that was done?

> This enhancement makes use of an unused bit in the EPT-PTE entry (vmx)
> and adjusts the definitions of get_entry() and set_entry() to carry a
> variable controlling this bit. It would probably have been better to add
> a new access type to p2m_access_t (which I tried), but in testing it
> caused subtle failures in the application using the mem-event API (the
> domains themselves seemed to work just fine though).

The above is even more so because this used bit is the only one left
at this point, i.e. I'd be rather careful giving this away without being
certain there's no alternative.

Jan


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