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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