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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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), please be sure such descriptions are accurate: There are no
functions with the names you mention, and vpmu_load() alters
last_vcpu only indirectly (via vpmu_save_force()).

>  void vpmu_destroy(struct vcpu *v)
>  {
>      struct vpmu_struct *vpmu = vcpu_vpmu(v);
>  
> +    if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_ALLOCATED) )
> +        return;

This appears to make unnecessary the respective checks in
amd_vpmu_destroy() and core2_vpmu_destroy().

Jan


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