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

["Kang, Luwei" <luwei.kang@xxxxxxxxx>]

> >>> 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().

Oh, yes. Thanks for your clarification.

Thanks,
Luwei Kang

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