Patch "xen/x86: don't lose event interrupts" 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

    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 <stable@vger.kernel.org> know about it.


>From c06b6d70feb32d28f04ba37aa3df17973fd37b6b Mon Sep 17 00:00:00 2001
From: Stefano Stabellini <sstabellini@kernel.org>
Date: Fri, 15 Apr 2016 18:23:00 -0700
Subject: xen/x86: don't lose event interrupts

From: Stefano Stabellini <sstabellini@kernel.org>

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.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Juergen Gross <jgross@suse.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -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 sstabellini@kernel.org are

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