This patch implements EOI suppression mode, where LAPIC on EOI command for level-triggered interrupt does not broadcast the EOI message to all APICs in the system. Instead, interrupt handler must follow LAPIC EOI with IOAPIC EOI. For modern IOAPICs, the later is done by writing to EOIR register. Otherwise, Intel provided Linux with a trick of temporary switching the pin config to edge and then back to level.
Documentation for EOIR exists only as part of ICH or PCH. Summary info for IOAPIC versions seems to exist in Linux source only.
I believe that suppression mode is useful even for single-IO-APIC configurations, since EOI is broadcast to all cores as well as IO-APICs.