Re: KVM Arm64 and Linux-RT issues

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Hi Sebastian,

On 8/13/19 1:58 PM, bigeasy@xxxxxxxxxxxxx wrote:
On 2019-07-27 14:37:11 [+0100], Julien Grall wrote:
8<------------
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -80,7 +80,7 @@ static inline bool userspace_irqchip(str
   static void soft_timer_start(struct hrtimer *hrt, u64 ns)
   {
   	hrtimer_start(hrt, ktime_add_ns(ktime_get(), ns),
-		      HRTIMER_MODE_ABS);
+		      HRTIMER_MODE_ABS_HARD);
   }

That's pretty neat, and matches the patch you already have for
x86. Feel free to add my

Acked-by: Marc Zyngier <maz@xxxxxxxxxx>

I can confirm the warning now disappeared. Feel free to added my tested-by:

Tested-by: Julien Grall <julien.grall@xxxxxxx>


|kvm_hrtimer_expire()
| kvm_timer_update_irq()
|   kvm_vgic_inject_irq()
|     vgic_lazy_init()
|                if (unlikely(!vgic_initialized(kvm))) {
|                 if (kvm->arch.vgic.vgic_model != KVM_DEV_TYPE_ARM_VGIC_V2)
|                         return -EBUSY;
|
|                 mutex_lock(&kvm->lock);

Is this possible path of any concern? This should throw a warning also
for !RT so probably not…

Hmmm, theoretically yes. In practice, it looks like the hrtimer will not be started before kvm_vcpu_first_run_init() is called on the first run.

The function will call kvm_vgic_map_resources() which will initialize the vgic if not already done.

Looking around, I think this is here to cater the case where KVM_IRQ_LINE is called before running.

I am not yet familiar with the vgic, so I may have missed something.


I prepared the patch below. This one could go straight to tglx's timer tree
since he has the _HARD bits there. I *think* it requires to set the bits
_HARD during _init() and _start() otherwise there is (or was) a warning…

Sebastian
8<------------

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date: Tue, 13 Aug 2019 14:29:41 +0200
Subject: [PATCH] KVM: arm/arm64: Let the timer expire in hardirq context on RT

The timers are canceled from an preempt-notifier which is invoked with
disabled preemption which is not allowed on PREEMPT_RT.
The timer callback is short so in could be invoked in hard-IRQ context
on -RT.

Let the timer expire on hard-IRQ context even on -RT.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Marc Zyngier <maz@xxxxxxxxxx>
Tested-by: Julien Grall <julien.grall@xxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
  virt/kvm/arm/arch_timer.c | 8 ++++----
  1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index 1be486d5d7cb4..0bfa7c5b5c890 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -80,7 +80,7 @@ static inline bool userspace_irqchip(struct kvm *kvm)
  static void soft_timer_start(struct hrtimer *hrt, u64 ns)
  {
  	hrtimer_start(hrt, ktime_add_ns(ktime_get(), ns),
-		      HRTIMER_MODE_ABS);
+		      HRTIMER_MODE_ABS_HARD);
  }
static void soft_timer_cancel(struct hrtimer *hrt)
@@ -697,11 +697,11 @@ void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu)
  	update_vtimer_cntvoff(vcpu, kvm_phys_timer_read());
  	ptimer->cntvoff = 0;
- hrtimer_init(&timer->bg_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
+	hrtimer_init(&timer->bg_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);
  	timer->bg_timer.function = kvm_bg_timer_expire;
- hrtimer_init(&vtimer->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
-	hrtimer_init(&ptimer->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
+	hrtimer_init(&vtimer->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);
+	hrtimer_init(&ptimer->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);
  	vtimer->hrtimer.function = kvm_hrtimer_expire;
  	ptimer->hrtimer.function = kvm_hrtimer_expire;

--
Julien Grall
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