linux-kernel - Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw

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Date:	Mon, 07 Feb 2011 16:38:55 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock

On 2011-02-07 16:15, Zachary Amsden wrote:
> On 02/07/2011 10:00 AM, Jan Kiszka wrote:
>> On 2011-02-07 15:11, Zachary Amsden wrote:
>>    
>>> On 02/07/2011 06:35 AM, Jan Kiszka wrote:
>>>      
>>>> On 2011-02-04 22:03, Zachary Amsden wrote:
>>>>
>>>>        
>>>>> On 02/04/2011 04:49 AM, Jan Kiszka wrote:
>>>>>
>>>>>          
>>>>>> Code under this lock requires non-preemptibility. Ensure this also over
>>>>>> -rt by converting it to raw spinlock.
>>>>>>
>>>>>>
>>>>>>            
>>>>> Oh dear, I had forgotten about that.  I believe kvm_lock might have the
>>>>> same assumption in a few places regarding clock.
>>>>>
>>>>>          
>>>> I only found a problematic section in kvmclock_cpufreq_notifier. Didn't
>>>> see this during my tests as I have CPUFREQ disabled in my .config.
>>>>
>>>> We may need something like this as converting kvm_lock would likely be
>>>> overkill:
>>>>
>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>> index 36f54fb..971ee0d 100644
>>>> --- a/arch/x86/kvm/x86.c
>>>> +++ b/arch/x86/kvm/x86.c
>>>> @@ -4530,7 +4530,7 @@ static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long va
>>>>    	struct cpufreq_freqs *freq = data;
>>>>    	struct kvm *kvm;
>>>>    	struct kvm_vcpu *vcpu;
>>>> -	int i, send_ipi = 0;
>>>> +	int i, me, send_ipi = 0;
>>>>
>>>>    	/*
>>>>    	 * We allow guests to temporarily run on slowing clocks,
>>>> @@ -4583,9 +4583,11 @@ static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long va
>>>>    		kvm_for_each_vcpu(i, vcpu, kvm) {
>>>>    			if (vcpu->cpu != freq->cpu)
>>>>    				continue;
>>>> +			me = get_cpu();
>>>>    			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>>>> -			if (vcpu->cpu != smp_processor_id())
>>>> +			if (vcpu->cpu != me)
>>>>    				send_ipi = 1;
>>>> +			put_cpu();
>>>>    		}
>>>>    	}
>>>>    	spin_unlock(&kvm_lock);
>>>>
>>>> Jan
>>>>
>>>>
>>>>        
>>> That looks like a good solution, and I do believe that is the only place
>>> the lock is used in that fashion - please add a comment though in the
>>> giant comment block above that preemption protection is needed for RT.
>>> Also, gcc should catch this, but moving the me variable into the
>>> kvm_for_each_vcpu loop should allow for better register allocation.
>>>
>>> The only other thing I can think of is that RT lock preemption may break
>>> some of the CPU initialization semantics enforced by kvm_lock if you
>>> happen to get a hotplug event just as the module is loading.  That
>>> should be rare, but if it is indeed a bug, it would be nice to fix, it
>>> would be a panic for sure not to initialize VMX.
>>>      
>> Hmm, is a cpu hotplug notifier allowed to run sleepy code? Can't
>> imagine. So we already have a strong reason to convert kvm_lock to a
>> raw_spinlock which obsoletes the above workaround.
>>    
> 
> I don't know as it is allowed to sleep, it doesn't call any sleeping 
> functions to my knowledge.  What worries me in the RT case is that the 
> spinlock acquired for hardware_enable might be preempted and run on 
> another CPU, which obviously isn't what you want.

I see now, there are calls to raw_smp_processor_id.

I think it's best to make this a raw lock. At this chance, some
read-only users of vm_list should be rcu'ified. Will have a look.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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