From a9f2c169366752927fd0bf8f5a9b78d3315f6c85 Mon Sep 17 00:00:00 2001 From: Nikita Leshenko Date: Sun, 5 Nov 2017 15:52:32 +0200 Subject: KVM: x86: ioapic: Clear Remote IRR when entry is switched to edge-triggered MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit [ Upstream commit a8bfec2930525808c01f038825d1df3904638631 ] 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 Reviewed-by: Liran Alon Signed-off-by: Konrad Rzeszutek Wilk Reviewed-by: Wanpeng Li Reviewed-by: Steve Rutherford Signed-off-by: Radim Krčmář Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- arch/x86/kvm/ioapic.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) (limited to 'arch') diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c index a7ac8688bba8..4b573c8694ac 100644 --- a/arch/x86/kvm/ioapic.c +++ b/arch/x86/kvm/ioapic.c @@ -306,8 +306,17 @@ 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); -- cgit v1.2.3