Re: [PATCH RFC v2 3/3] x86/altp2m: p2m_altp2m_propagate_change() should honor present page order

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

> I may be entirely wrong and hence that part of the change may also be
> wrong, but I'm having trouble seeing why the original
> "!mfn_eq(m, INVALID_MFN)" wasn't "mfn_eq(m, INVALID_MFN)". Isn't the
> goal there to pre-fill entries that were previously invalid, instead of
> undoing prior intentional divergence from the host P2M? (I have
> intentionally not reflected this aspect in the description yet; I can't
> really write a description of this without understanding what's going on
> in case the original code was correct.)

This function only gets called from p2m-ept when the hostp2m gets an
update. In that case this check goes through all altp2m's to see if
any of them has an entry for what just got changed in the host, and
overwrites the altp2m with that from the host. If there is no entry in
the altp2m it doesn't pre-populate. That should only happen if the
altp2m actually needs it and runs into a pagefault. So it is correct
as-is, albeit being a subtle (and undocumented) behavior of the
hostp2m and its effect on the altp2m's. But that's why we never
actually make any changes on the hostp2m, we always create an altp2m
and apply changes (mem_access/remapping) there.

Tamas