Re: [Xen-devel] [PATCH v3] x86/VPMU: Clear last_vcpu when destroying VPMU

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/17/2014 11:21 AM, Konrad Rzeszutek Wilk wrote:
> On Wed, Dec 17, 2014 at 10:35:47AM -0500, Boris Ostrovsky 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 atomically since otherwise there is a (somewhat
> > 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.
> >
> > We should also check for VPMU_CONTEXT_ALLOCATED in vpmu_destroy() to
> > avoid unnecessary cmpxchg() and arch-specific destroy ops. Thus
> > checks in AMD and Intel routines are no longer needed.
> >
> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > ---
> >   xen/arch/x86/hvm/svm/vpmu.c       |    3 ---
> >   xen/arch/x86/hvm/vmx/vpmu_core2.c |    2 --
> >   xen/arch/x86/hvm/vpmu.c           |    7 +++++++
> >   3 files changed, 7 insertions(+), 5 deletions(-)
> >
> > Changes in v3:
> >   * Use cmpxchg instead of IPI
> >   * Use correct routine nemas in commit message
> >   * Remove duplicate test for VPMU_CONTEXT_ALLOCATED in arch-specific
> >     destroy ops
> >
> > Changes in v2:
> >   * Test last_vcpu locally before IPI
> >   * Don't handle local pcpu as a special case --- on_selected_cpus
> >     will take care of that
> >   * Dont't cast variables unnecessarily
> >
> > diff --git a/xen/arch/x86/hvm/svm/vpmu.c b/xen/arch/x86/hvm/svm/vpmu.c
> > index 8e07a98..4c448bb 100644
> > --- a/xen/arch/x86/hvm/svm/vpmu.c
> > +++ b/xen/arch/x86/hvm/svm/vpmu.c
> > @@ -403,9 +403,6 @@ static void amd_vpmu_destroy(struct vcpu *v)
> >   {
> >       struct vpmu_struct *vpmu = vcpu_vpmu(v);
> >   
> > -    if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_ALLOCATED) )
> > -        return;
> > -
> >       if ( ((struct amd_vpmu_context *)vpmu->context)->msr_bitmap_set )
> >           amd_vpmu_unset_msr_bitmap(v);
> >   
> > diff --git a/xen/arch/x86/hvm/vmx/vpmu_core2.c b/xen/arch/x86/hvm/vmx/vpmu_core2.c
> > index 68b6272..590c2a9 100644
> > --- a/xen/arch/x86/hvm/vmx/vpmu_core2.c
> > +++ b/xen/arch/x86/hvm/vmx/vpmu_core2.c
> > @@ -818,8 +818,6 @@ static void core2_vpmu_destroy(struct vcpu *v)
> >       struct vpmu_struct *vpmu = vcpu_vpmu(v);
> >       struct core2_vpmu_context *core2_vpmu_cxt = vpmu->context;
> >   
> > -    if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_ALLOCATED) )
> > -        return;
> >       xfree(core2_vpmu_cxt->pmu_enable);
> >       xfree(vpmu->context);
> >       if ( cpu_has_vmx_msr_bitmap )
> > diff --git a/xen/arch/x86/hvm/vpmu.c b/xen/arch/x86/hvm/vpmu.c
> > index 1df74c2..7cc95ae 100644
> > --- a/xen/arch/x86/hvm/vpmu.c
> > +++ b/xen/arch/x86/hvm/vpmu.c
> > @@ -250,6 +250,13 @@ void vpmu_initialise(struct vcpu *v)
> >   void vpmu_destroy(struct vcpu *v)
> >   {
> >       struct vpmu_struct *vpmu = vcpu_vpmu(v);
> > +    struct vcpu **last = &per_cpu(last_vcpu, vpmu->last_pcpu);
> > +
> > +    if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_ALLOCATED) )
> > +        return;
> > +
> Could this just be:
>
>         (void)cmpxchg(&per_cpu(last_vcpu, vpmu->last_pcpu), v, NULL);
>
> so that we don't do the 'per_cpu' access in case we return because
> VPMU_CONTEXT_ALLOCATED is not set?

Ugh. This is actually what I meant to send but I forgot to refresh the 
patch.
Do you want me to re-send it?

-boris


> Either way,
>
> Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>
> Thank you for spotting this.
> > +    /* Need to clear last_vcpu in case it points to v */
> > +    (void)cmpxchg(last, v, NULL);
> >   
> >       if ( vpmu->arch_vpmu_ops && vpmu->arch_vpmu_ops->arch_vpmu_destroy )
> >           vpmu->arch_vpmu_ops->arch_vpmu_destroy(v);
> > --
> > 1.7.1

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