linux-kernel - [PATCH 3.10 258/268] xen/x86: don't lose event interrupts

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Date:   Mon, 19 Jun 2017 20:32:37 +0200
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Julia Lawall <julia.lawall@...6.fr>,
        Jiri Slaby <jslaby@...e.cz>, Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 258/268] xen/x86: don't lose event interrupts

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: Jiri Slaby <jslaby@...e.cz>
Signed-off-by: Willy Tarreau <w@....eu>
---
 arch/x86/xen/time.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 13e8935..e3600eb 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -338,11 +338,11 @@ static int xen_vcpuop_set_next_event(unsigned long delta,
 	WARN_ON(evt->mode != CLOCK_EVT_MODE_ONESHOT);
 
 	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;
 }
-- 
2.8.0.rc2.1.gbe9624a