Re: KVM Arm64 and Linux-RT issues

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 24 Jul 2019, Marc Zyngier wrote:
> On 23/07/2019 18:58, Julien Grall wrote:
> It really feels like a change in hrtimer_cancel semantics. From what I
> understand, this is used to avoid racing against the softirq, but boy it
> breaks things.
> 
> If this cannot be avoided, this means we can't cancel the background
> timer (which is used to emulate the vcpu timer while it is blocked
> waiting for an interrupt), then we must move this canceling to the point
> where the vcpu is unblocked (instead of scheduled), which may have some
> side effects -- I'll have a look.
> 
> But that's not the only problem: We also have hrtimers used to emulate
> timers while the vcpu is running, and these timers are canceled in
> kvm_timer_vcpu_put(), which is also called from a preempt notifier.
> Unfortunately, I don't have a reasonable solution for that (other than
> putting this hrtimer_cancel in a workqueue and start chasing the
> resulting races).

The fix is simple. See below. We'll add that to the next RT release. That
will take a while as I'm busy with posting RT stuff for upstream :)

Thanks,

	tglx

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);
 }
 
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