Re: KVM Arm64 and Linux-RT issues

From: Marc Zyngier <maz@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "linux-rt-users@vger.kernel.org" <linux-rt-users@vger.kernel.org>,
	"bigeasy@linutronix.de" <bigeasy@linutronix.de>,
	Julien Grall <julien.grall@arm.com>,
	anna-maria@linutronix.de,
	"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>
Subject: Re: KVM Arm64 and Linux-RT issues
Date: Sat, 27 Jul 2019 12:13:38 +0100	[thread overview]
Message-ID: <86zhkzn319.wl-maz@kernel.org> (raw)
In-Reply-To: <alpine.DEB.2.21.1907270053360.1791@nanos.tec.linutronix.de>

Hi Thomas,

On Fri, 26 Jul 2019 23:58:38 +0100,
Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> 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 :)

Ah, thanks for that! And yes, looking forward to RT upstream, it's
just about time! ;-)

> 
> 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);
>  }
>  

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

Acked-by: Marc Zyngier <maz@kernel.org>

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.
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