Re: [Xen-devel] [PATCH v2 for 4.5] x86/VPMU: Clear last_vcpu when destroying VPMU

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/16/2014 04:31 AM, Jan Beulich wrote:
> > > > On 15.12.14 at 23:24, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > We need to make sure that last_vcpu is not pointing to VCPU whose
> > VPMU is being destroyed. Otherwise we may try to dereference it in
> > the future, when VCPU is gone.
> >
> > We have to do this via IPI since otherwise there is a (somewheat
> > theoretical) chance that between test and subsequent clearing
> > of last_vcpu the remote processor (i.e. vpmu->last_pcpu) might do
> > both load_vpmu() and then save_vpmu() for another VCPU. The former
> > will clear last_vcpu and the latter will set it to something else.
> Apart from the question of using cmpxchg instead of the IPI (I can
> see with the current model that using an IPI is the only clean way,
> i.e. the alternative - if usable - would mean altering existing logic
> too),

You mean something like

    struct vcpu **last = &per_cpu(last_vcpu, vpmu->last_pcpu);
    cmpxchg(last, v, NULL);

Yes, that could work.

-boris



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