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SubjectRe: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock
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.

Cheers,

Zach


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