[PATCH 2/3] KVM: arm64: nv: Emulate ISTATUS when emulated timers are fired.

[Ganapatrao Kulkarni]

Guest-Hypervisor forwards the timer interrupt to Guest-Guest, if it is
enabled, unmasked and ISTATUS bit of register CNTV_CTL_EL0 is set for a
loaded timer.

For NV2 implementation, the Host-Hypervisor is not emulating the ISTATUS
bit while forwarding the Emulated Vtimer Interrupt to Guest-Hypervisor.
This results in the drop of interrupt from Guest-Hypervisor, where as
Host Hypervisor marked it as an active interrupt and expecting Guest-Guest
to consume and acknowledge. Due to this, some of the Guest-Guest vCPUs
are stuck in Idle thread and rcu soft lockups are seen.

This issue is not seen with NV1 case since the register CNTV_CTL_EL0 read
trap handler is emulating the ISTATUS bit.

Adding code to set/emulate the ISTATUS when the emulated timers are fired.

Signed-off-by: Ganapatrao Kulkarni <gankulkarni at os.amperecomputing.com>
---
 arch/arm64/kvm/arch_timer.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
index 27a6ec46803a..0b32d943d2d5 100644
--- a/arch/arm64/kvm/arch_timer.c
+++ b/arch/arm64/kvm/arch_timer.c
@@ -63,6 +63,7 @@ static u64 kvm_arm_timer_read(struct kvm_vcpu *vcpu,
 			      struct arch_timer_context *timer,
 			      enum kvm_arch_timer_regs treg);
 static bool kvm_arch_timer_get_input_level(int vintid);
+static u64 read_timer_ctl(struct arch_timer_context *timer);
 
 static struct irq_ops arch_timer_irq_ops = {
 	.get_input_level = kvm_arch_timer_get_input_level,
@@ -356,6 +357,8 @@ static enum hrtimer_restart kvm_hrtimer_expire(struct hrtimer *hrt)
 		return HRTIMER_RESTART;
 	}
 
+	/* Timer emulated, emulate ISTATUS also */
+	timer_set_ctl(ctx, read_timer_ctl(ctx));
 	kvm_timer_update_irq(vcpu, true, ctx);
 	return HRTIMER_NORESTART;
 }
@@ -458,6 +461,8 @@ static void timer_emulate(struct arch_timer_context *ctx)
 	trace_kvm_timer_emulate(ctx, should_fire);
 
 	if (should_fire != ctx->irq.level) {
+		/* Timer emulated, emulate ISTATUS also */
+		timer_set_ctl(ctx, read_timer_ctl(ctx));
 		kvm_timer_update_irq(ctx->vcpu, should_fire, ctx);
 		return;
 	}
-- 
2.33.1