Re: [PATCH v3 4/4] KVM: x86: Add support for local interrupt requests from userspace

[Steve Rutherford <srutherford@google.com>]

> However, why is the roundtrip to userspace necessary?  Could you pass
> the extint index directly as an argument to KVM_INTERRUPT?  It's
> backwards-compatible, because KVM_INTERRUPT so far could not be used
> together with an in-kernel LAPIC.  If you could do that, you could also
> avoid the new userspace_extint_available field.

Implemented a basic version of this, and ran into some potential
issues with this strategy. Supporting PIC masking/unmasking by the
CPU/APIC means that either:
A) PIC interrupts need to be bufferable in the kernel (with some way
   of comparing priorities).
B) the APIC state needs to be read in order to fetch the bit as to
   whether or not the PIC is being masked (which I believe can be done
   from userspace via the APIC state ioctl).
C) something hacky that doesn't conform to the PIC spec but still
   happens to boot common OSes (like buffering the interrupts and
   injecting them in the order of arrival (which is wrong)).

Steve