The previous code unconditionally required a legacy INTX line, which
on bare metal is missing for nearly every modern 82599/X540/X550
board that has its IRQ lines wired to MSI-X only. pcid_interface
allocates MSI-X when the device advertises it, falls back to MSI,
then to legacy INTX, so the change preserves every existing path
while unlocking the much higher line-rate headroom of MSI-X
per-vector queuing that the device's enable_msix_interrupt code
already implements in device.rs.
The IRQ-acknowledge-and-write pattern is unchanged: the kernel
masks the line on delivery, we must re-arm unconditionally or a
foreign interrupt on a shared line leaves the line masked forever.
Red Bear OS
717bc436be