linux-kernel - [PATCH 4.4 15/43] xen/x86: dont lose event interrupts

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Message-Id: <20170501212600.141900530@linuxfoundation.org>
Date:   Mon,  1 May 2017 14:27:15 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Julia Lawall <julia.lawall@...6.fr>
Subject: [PATCH 4.4 15/43] xen/x86: dont lose event interrupts

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Stefano Stabellini <sstabellini@...nel.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@...nel.org>
Acked-by: Juergen Gross <jgross@...e.com>
Cc: Julia Lawall <julia.lawall@...6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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;
 }