[RFC PATCH 0/2] Expose available KVM free memory slot count to help avoid aborts

[Alex Williamson <alex.williamson@redhat.com>]

When doing device assignment, we use cpu_register_physical_memory() to
directly map the qemu mmap of the device resource into the address
space of the guest.  The unadvertised feature of the register physical
memory code path on kvm, at least for this type of mapping, is that it
needs to allocate an index from a small, fixed array of memory slots.
Even better, if it can't get an index, the code aborts deep in the
kvm specific bits, preventing the caller from having a chance to
recover.

It's really easy to hit this by hot adding too many assigned devices
to a guest (pretty easy to hit with too many devices at instantiation
time too, but the abort is slightly more bearable there).

I'm assuming it's pretty difficult to make the memory slot array
dynamically sized.  If that's not the case, please let me know as
that would be a much better solution.

I'm not terribly happy with the solution in this series, it doesn't
provide any guarantees whether a cpu_register_physical_memory() will
succeed, only slightly better educated guesses.

Are there better ideas how we could solve this?  Thanks,

Alex

---

Alex Williamson (2):
      device-assignment: Count required kvm memory slots
      kvm: Allow querying free slots


 hw/device-assignment.c |   59 +++++++++++++++++++++++++++++++++++++++++++++++-
 hw/device-assignment.h |    3 ++
 kvm-all.c              |   16 +++++++++++++
 kvm.h                  |    2 ++
 4 files changed, 79 insertions(+), 1 deletions(-)