Patch "KVM: x86: ioapic: Clear Remote IRR when entry is switched to edge-triggered" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-x86-ioapic-clear-remote-irr-when-entry-is-switched-to-edge-triggered.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From foo@baz Thu Feb  1 14:14:46 CET 2018
From: Nikita Leshenko <nikita.leshchenko@oracle.com>
Date: Sun, 5 Nov 2017 15:52:32 +0200
Subject: KVM: x86: ioapic: Clear Remote IRR when entry is switched to edge-triggered

From: Nikita Leshenko <nikita.leshchenko@oracle.com>


[ 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 <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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/kvm/ioapic.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/ioapic.c
+++ b/arch/x86/kvm/ioapic.c
@@ -296,8 +296,17 @@ static void ioapic_write_indirect(struct
 		} 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);


Patches currently in stable-queue which might be from nikita.leshchenko@oracle.com are

queue-4.4/kvm-x86-don-t-re-execute-instruction-when-not-passing-cr2-value.patch
queue-4.4/kvm-x86-ioapic-preserve-read-only-values-in-the-redirection-table.patch
queue-4.4/kvm-x86-ioapic-fix-level-triggered-eoi-and-ioapic-reconfigure-race.patch
queue-4.4/kvm-x86-emulator-return-to-user-mode-on-l1-cpl-0-emulation-failure.patch
queue-4.4/kvm-x86-ioapic-clear-remote-irr-when-entry-is-switched-to-edge-triggered.patch