[Xen-devel] assumptions when hvm guest uses string instructions on MMIO memory

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

I'm wondering if the assumptions currently made are appropriate:

- movs assumes that either address is not in MMIO space (What if the
  guest uses it e.g. on video memory for scrolling?)
- stos and lods assume that the mmio memory is physically contiguous,
  while movs doesn't (and even ins/outs for PIO don't, although I'm
  not clear why, as it still seems to be assumed that the memory
  accessed is not in MMIO space)

Likewise I find it at least strange that all the I/O related
hvm_copy_{from,to}_guest_virt invocations have their return value
cast to void instead of forcing page faults into the guest. While I
can see the point for single datum instructions (the CPU supposedly
did the checking, except perhaps for ins/outs), movs where the
non-mmio address crosses a page boundary and lods/stos because
they're not being broken up would still seem to cause issues. Even
in the single datum case I think it would be much more consistent
to force a fault into the guest rather than silently ignoring any
problems.

Thanks, Jan

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