I just don't want to end up with something like:
(GICV3 && ARM64) || (GICV3 && ARM && KERNEL>4.4) || (SuperIRQC && i986)
or
(ARM || ARM64) && HAS_IRQ_ROUTING

Instead: If the kernel needs it, it tells you. Full stop.

 Agree.

To be honest it's me to blame here to not having introduced the
capability earlier. At the moment ARM has a different code path for
KVM_SIGNAL_MSI, which does not bail out if the flag field is set. With
Eric's patches this changes and we use the irqchip.c generic code, which
returns -EINVAL atm. So I plan to introduce this capability already with
the ITS emulation series, so we can just pick it up in the IRQ routing
series.

 Then may be you follow https://lkml.org/lkml/2015/7/7/115 and replace flag with something like
KVM_SIGNAL_EXT_MSI ioctl ? After all you were one of people who voted against using flags.

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia