... > 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
