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