...

With this patch series, it's actually even not possible for userspace VMM
to allocate private page by a direct write, it's basically unmapped from
there. If it really wants to, it should so something special, by intention,
that's basically the conversion, which we should allow.

A VM can pass GPA backed by private pages to userspace VMM and when
Userspace VMM accesses the backing hva there will be pages allocated
to back the shared fd causing 2 sets of pages backing the same guest
memory range.

Thanks for bringing this up. But in my mind I still think userspace VMM
can do and it's its responsibility to guarantee that, if that is hard
required. By design, userspace VMM is the decision-maker for page
conversion and has all the necessary information to know which page is
shared/private. It also has the necessary knobs to allocate/free the
physical pages for guest memory. Definitely, we should make userspace
VMM more robust.

Making Userspace VMM more robust to avoid double allocation can get
complex, it will have to keep track of all in-use (by Userspace VMM)
shared fd memory to disallow conversion from shared to private and
will have to ensure that all guest supplied addresses belong to shared
GPA ranges.
A coarser but simpler alternative could be to always allow shared to
private conversion with unbacking the memory from shared fd and exit
if the VMM runs in double allocation scenarios. In either cases,
unbacking shared fd memory ideally should prevent memory allocation on
subsequent write accesses to ensure double allocation scenarios are
caught early.

Regards,
Vishal