Why can't _PAGE_PROTNONE be _PAGE_ACCESSED?

Why can't _PAGE_PROTNONE be _PAGE_ACCESSED?

[Jan Beulich]

With the overlaying of _PAGE_PSE and _PAGE_PAT there is a latent issue
with the definition of _PAGE_PROTNONE: As soon as _PAGE_PAT gets
actively used, it'll need to become part of _PAGE_CHG_MASK, and hence
_PAGE_PROTNONE cannot use _PAGE_PSE anymore. One option certainly
is to use one of the two remaining unused (available) bits, but it would
seem to me that simply using _PAGE_ACCESSED here ought to work - but
perhaps I'm overlooking some critical aspect?

The background of this is that Xen's dom0 (and similarly DomU-s that have
physical devices assigned) cannot change the setting in PAT_MSR, and
hence has to live with being required to use _PAGE_PAT for establishing
WC mappings. That'll be a requirement for Dom0 and pv pass-through
support in the mainline tree (whenever that gets enabled) anyway, but
I'll have to do it eagerly in our forward-ported Xen tree, which is why I'm
trying to settle early on the route to take here.

Thanks, Jan