Patch "xen/x86: don't lose event interrupts" has been added to the 4.4-stable tree

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

xen/x86: don't lose event interrupts

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:
xen-x86-don-t-lose-event-interrupts.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 <[email protected]> know about it.

From: Stefano Stabellini <[email protected]>
Date: Fri, 15 Apr 2016 18:23:00 -0700
Subject: xen/x86: don't lose event interrupts

From: Stefano Stabellini <[email protected]>

commit c06b6d70feb32d28f04ba37aa3df17973fd37b6b upstream.

On slow platforms with unreliable TSC, such as QEMU emulated machines, it is possible for the kernel to request the next event in the past. In that case, in the current implementation of xen_vcpuop_clockevent, we simply return -ETIME. To be precise the Xen returns -ETIME and we pass it on. However the result of this is a missed event, which simply causes the kernel to hang.

Instead it is better to always ask the hypervisor for a timer event, even if the timeout is in the past. That way there are no lost interrupts and the kernel survives. To do that, remove the VCPU_SSHOTTMR_future flag.

---
 arch/x86/xen/time.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

@@ -343,11 +343,11 @@ static int xen_vcpuop_set_next_event(uns
 	WARN_ON(!clockevent_state_oneshot(evt));
 
 	single.timeout_abs_ns = get_abs_timeout(delta);
-	single.flags = VCPU_SSHOTTMR_future;
+	/* Get an event anyway, even if the timeout is already expired */
+	single.flags = 0;
 
 	ret = HYPERVISOR_vcpu_op(VCPUOP_set_singleshot_timer, cpu, &single);
-
-	BUG_ON(ret != 0 && ret != -ETIME);
+	BUG_ON(ret != 0);
 
 	return ret;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/xen-x86-don-t-lose-event-interrupts.patch