Re: [Xen-devel] [PATCH v2] x86/vpmu: add cpu hot unplug notifier for vpmu

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.05.17 at 16:23, <luwei.kang@xxxxxxxxx> wrote:
>> > --- a/xen/arch/x86/cpu/vpmu.c
>> > +++ b/xen/arch/x86/cpu/vpmu.c
>> > @@ -859,6 +859,7 @@ static int cpu_callback(
>> >      {
>> >          vpmu_save_force(vcpu);
>> >          vpmu_reset(vpmu, VPMU_CONTEXT_LOADED);
>> > +        per_cpu(last_vcpu, cpu) = NULL;        // OR: this_cpu(last_vcpu) 
> = NULL;
>> >      }
>> >     As you mentioned in before comments, it has been done in 
> vpmu_save_force(). So this change is unnecessary.
>> 
>> Indeed. But all I was talking is last_pcpu (whereas you once again talk 
> about last_vcpu).
>> 
>> >     In summary, I think it is enough to solve the issue in vpmu_load() and 
> vpmu_arch_destroy().
>> 
>> That's what I alluded to in my reply.
>> 
>> >     After cpu_callback() function, per_cpu(last_vcpu, vpmu->last_pcpu)
>> > will be NULL
>> 
>> No. per_cpu(..., <offlined-pcpu>) simply is invalid.
>> 
>> > and VPMU_CONTEXT_LOADED will be clear.
>> >     In vpmu_arch_destroy(), there will not make remote call to clear last.
>> 
>> I don't understand this sentence.
> 
> I mean per_cpu(..., <offlined-pcpu>) will be NULL after cpu_callback(), so 
> that 
> "per_cpu(last_vcpu, vpmu->last_pcpu) == v" check in 
> vpmu_arch_destroy() will be fail when last_pcpu is the offlined pCPU. Or, it 
> may make a remote call to clear the last_vpcu (vpmu_clear_last()).
> But I don't understand why simply is invalid, last_vcpu set to NULL is 
> presented in source code.  How to comprehend it?

per_cpu(..., <offlined-pcpu>) will fault once the CPU is actually
offline. See free_percpu_area().

Jan


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