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

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Date:	Mon, 07 Feb 2011 10:15:43 -0500
From:	Zachary Amsden <zamsden@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.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 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.

Zach
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