Re: [Xen-devel] [PATCH v4 for 4.5] x86/VPMU: Clear last_vcpu when destroying VPMU

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.12.14 at 19:06, <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 vpmu_load() and then vpmu_save() for another VCPU. The former
> will clear last_vcpu and the latter will set it to something else.
> 
> Performing this operation via IPI will guarantee that nothing can
> happen on the remote processor between testing and clearing of
> last_vcpu.
> 
> We should also check for VPMU_CONTEXT_ALLOCATED in vpmu_destroy() to
> avoid unnecessary percpu tests and arch-specific destroy ops. Thus
> checks in AMD and Intel routines are no longer needed.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>


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