Re: KVM Arm64 and Linux-RT issues

From: Julien Grall <julien.grall@arm.com>
To: Marc Zyngier <maz@kernel.org>, Thomas Gleixner <tglx@linutronix.de>
Cc: "linux-rt-users@vger.kernel.org" <linux-rt-users@vger.kernel.org>,
	"bigeasy@linutronix.de" <bigeasy@linutronix.de>,
	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 14:37:11 +0100	[thread overview]
Message-ID: <960b5ed1-6d0f-3cee-da37-6061b4946c1a@arm.com> (raw)
In-Reply-To: <86zhkzn319.wl-maz@kernel.org>

Hi,

On 7/27/19 12:13 PM, Marc Zyngier wrote:
> 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>

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

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

Thank you both for the help!

Cheers,

-- 
Julien Grall
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