Commit a8bfec29 authored Nov 05, 2017 by Nikita Leshenko Committed by Paolo Bonzini Nov 17, 2017

KVM: x86: ioapic: Clear Remote IRR when entry is switched to edge-triggered



Some OSes (Linux, Xen) use this behavior to clear the Remote IRR bit for
IOAPICs without an EOI register. They simulate the EOI message manually
by changing the trigger mode to edge and then back to level, with the
entry being masked during this.

QEMU implements this feature in commit ed1263c363c9
("ioapic: clear remote irr bit for edge-triggered interrupts")

As a side effect, this commit removes an incorrect behavior where Remote
IRR was cleared when the redirection table entry was rewritten. This is not
consistent with the manual and also opens an opportunity for a strange
behavior when a redirection table entry is modified from an interrupt
handler that handles the same entry: The modification will clear the
Remote IRR bit even though the interrupt handler is still running.

Signed-off-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Wanpeng Li <wanpeng.li@hotmail.com>
Reviewed-by: Steve Rutherford <srutherford@google.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

parent 7d225368

arch/x86/kvm/ioapic.c

+10 −1

Original line number	Diff line number	Diff line
		@@ -304,8 +304,17 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
		} else {
		e->bits &= ~0xffffffffULL;
		e->bits \|= (u32) val;
		e->fields.remote_irr = 0;
		}

		/*
		* Some OSes (Linux, Xen) assume that Remote IRR bit will
		* be cleared by IOAPIC hardware when the entry is configured
		* as edge-triggered. This behavior is used to simulate an
		* explicit EOI on IOAPICs that don't have the EOI register.
		*/
		if (e->fields.trig_mode == IOAPIC_EDGE_TRIG)
		e->fields.remote_irr = 0;

		mask_after = e->fields.mask;
		if (mask_before != mask_after)
		kvm_fire_mask_notifiers(ioapic->kvm, KVM_IRQCHIP_IOAPIC, index, mask_after);