[PATCH v2 char-misc 1/1] x86/hyperv: Fix PIT shutdown quirk

[Michael Kelley <mikelley@microsoft.com>]

pit_shutdown() doesn't work on Hyper-V because of a quirk in the
PIT emulation. After shutdown the emulated PIT continues to interrupt
@18.2 HZ. This problem exists in all versions of Hyper-V and just
had not previously been noticed. So replace the normal PIT shutdown
function with an alternate version that works on Hyper-V.

The alternate shutdown function could go in the i8253.c module,
but it seems better to keep the hack with the Hyper-V code rather
than clutter up i8253.c with #ifdef CONFIG_HYPERV.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
---
Changes in v2:
* Fixed commit message and comment in the code to indicate
  that the PIT continues to interrupt @18.2 HZ, not every
  18.2 seconds. [Dexuan Cui]

---
 arch/x86/hyperv/hv_init.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 7abb09e..514fcbe 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -32,6 +32,9 @@
 #include <linux/slab.h>
 #include <linux/cpuhotplug.h>
 
+#include <linux/io.h>
+#include <linux/i8253.h>
+
 #ifdef CONFIG_HYPERV_TSCPAGE
 
 static struct ms_hyperv_tsc_page *tsc_pg;
@@ -97,6 +100,26 @@ static u64 read_hv_clock_msr(struct clocksource *arg)
 
 u32 hv_max_vp_index;
 
+#if defined(CONFIG_CLKEVT_I8253)
+/*
+ * Hyper-V emulation of the PIT has a quirk such that the
+ * normal i8253 pit_shutdown() routine doesn't stop the PIT.
+ * It keeps interrupting @18.2 HZ.  This alternate
+ * shutdown routine mirrors the normal one, but does not
+ * set the counter value to zero.
+ */
+static int hyperv_pit_shutdown(struct clock_event_device *evt)
+{
+	if (!clockevent_state_oneshot(evt) && !clockevent_state_periodic(evt))
+		return 0;
+
+	raw_spin_lock(&i8253_lock);
+	outb_p(0x30, PIT_MODE);
+	raw_spin_unlock(&i8253_lock);
+	return 0;
+}
+#endif
+
 static int hv_cpu_init(unsigned int cpu)
 {
 	u64 msr_vp_index;
@@ -325,6 +348,14 @@ void __init hyperv_init(void)
 	if (cpuhp < 0)
 		goto free_vp_assist_page;
 
+#if defined(CONFIG_CLKEVT_I8253)
+	/*
+	 * Override the PIT shutdown routine due to a quirk in
+	 * Hyper-V emulation of the PIT.
+	 */
+	i8253_clockevent.set_state_shutdown = hyperv_pit_shutdown;
+#endif
+
 	/*
 	 * Setup the hypercall page and enable hypercalls.
 	 * 1. Register the guest ID
-- 
1.8.3.1