[PATCH] KVM: lapic: restart counter on change to periodic mode

[Matt delco <delco@google.com>]

From: Matt Delco <delco@google.com>

Time seems to eventually stop in a Windows VM when using Skype.
Instrumentation shows that the OS is frequently switching the APIC
timer between one-shot and periodic mode.  The OS is typically writing
to both LVTT and TMICT.  When time stops the sequence observed is that
the APIC was in one-shot mode, the timer expired, and the OS writes to
LVTT (but not TMICT) to change to periodic mode.  No future timer events
are received by the OS since the timer is only re-armed on TMICT writes.

With this change time continues to advance in the VM.  TBD if physical
hardware will reset the current count if/when the mode is changed to
period and the current count is zero.

Signed-off-by: Matt Delco <delco@google.com>
---
 arch/x86/kvm/lapic.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 685d17c11461..fddd810eeca5 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1935,14 +1935,19 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 
 		break;
 
-	case APIC_LVTT:
+	case APIC_LVTT: {
+		u32 timer_mode = apic->lapic_timer.timer_mode;
 		if (!kvm_apic_sw_enabled(apic))
 			val |= APIC_LVT_MASKED;
 		val &= (apic_lvt_mask[0] | apic->lapic_timer.timer_mode_mask);
 		kvm_lapic_set_reg(apic, APIC_LVTT, val);
 		apic_update_lvtt(apic);
+		if (timer_mode == APIC_LVT_TIMER_ONESHOT &&
+		    apic_lvtt_period(apic) &&
+		    !hrtimer_active(&apic->lapic_timer.timer))
+			start_apic_timer(apic);
 		break;
-
+	}
 	case APIC_TMICT:
 		if (apic_lvtt_tscdeadline(apic))
 			break;
-- 
2.23.0.rc1.153.gdeed80330f-goog