linux-kernel - Re: [PATCH] KVM: arm/arm64: don't set vtimer->cnt_ctl in kvm_arch_timer

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Message-ID: <daece017-50be-494f-4176-6e5f081edfa2@arm.com>
Date:   Wed, 13 Dec 2017 09:27:43 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Auger Eric <eric.auger@...hat.com>, Jia He <hejianet@...il.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu
Cc:     Jia He <jia.he@...-semitech.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm/arm64: don't set vtimer->cnt_ctl in
 kvm_arch_timer_handler

On 13/12/17 09:08, Auger Eric wrote:
> Marc,
> On 13/12/17 09:56, Marc Zyngier wrote:
>> Hi Jia,
>>
>> On 13/12/17 07:00, Jia He wrote:
>>> In our Armv8a server (qualcomm Amberwing, non VHE), after applying
>>> Christoffer's timer optimizing patchset(Optimize arch timer register
>>> handling), the guest is hang during kernel booting.
>>>
>>> The error root cause might be as follows:
>>> 1. in kvm_arch_timer_handler, it reset vtimer->cnt_ctl with current
>>> cntv_ctl register value. And then it missed some cases to update timer's
>>> irq (irq.level) when kvm_timer_irq_can_fire() is false
>>> 2. It causes kvm_vcpu_check_block return 0 instead of -EINTR
>>> 	kvm_vcpu_check_block
>>> 		kvm_cpu_has_pending_timer
>>> 			kvm_timer_is_pending
>>> 				kvm_timer_should_fire
>>> 3. Thus, the kvm hyp code can not break the loop in kvm_vcpu_block (halt
>>> poll process) and the guest is hang forever
>>>
>>> Fixes: b103cc3f10c0 ("KVM: arm/arm64: Avoid timer save/restore in vcpu entry/exit")
>>> Signed-off-by: Jia He <jia.he@...-semitech.com>
>>> ---
>>>  virt/kvm/arm/arch_timer.c | 1 -
>>>  1 file changed, 1 deletion(-)
>>>
>>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
>>> index f9555b1..bb86433 100644
>>> --- a/virt/kvm/arm/arch_timer.c
>>> +++ b/virt/kvm/arm/arch_timer.c
>>> @@ -100,7 +100,6 @@ static irqreturn_t kvm_arch_timer_handler(int irq, void *dev_id)
>>>  	vtimer = vcpu_vtimer(vcpu);
>>>  
>>>  	if (!vtimer->irq.level) {
>>> -		vtimer->cnt_ctl = read_sysreg_el0(cntv_ctl);
>>>  		if (kvm_timer_irq_can_fire(vtimer))
>>>  			kvm_timer_update_irq(vcpu, true, vtimer);
>>>  	}
>>>
>>
>> Which patches are you looking at? The current code in mainline looks
>> like this:
>>
>> 	vtimer = vcpu_vtimer(vcpu);
>>
>> 	vtimer->cnt_ctl = read_sysreg_el0(cntv_ctl);
>> 	if (kvm_timer_irq_can_fire(vtimer))
>> 		kvm_timer_update_irq(vcpu, true, vtimer);
>>
>> I'd suggest you use mainline and report if this doesn't work
> the removal of if (!vtimer->irq.level) test happened in:
> [PATCH v7 3/8] KVM: arm/arm64: Don't cache the timer IRQ level
> 
> which is not upstream.
Ah, my bad (I have that series in my working tree already...).

I still think Jia's approach to this is not quite right. If you don't
update the status of the timer by reading the HW value, how can you
decide whether the timer can fire or not?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...