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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
> Sent: Tuesday, January 06, 2015 12:33 AM
> 
> On Thu, Dec 18, 2014 at 01:06:40PM -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 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>
> > ---
> >  xen/arch/x86/hvm/svm/vpmu.c       |    3 ---
> >  xen/arch/x86/hvm/vmx/vpmu_core2.c |    2 --
> >  xen/arch/x86/hvm/vpmu.c           |   20 ++++++++++++++++++++
> >  3 files changed, 20 insertions(+), 5 deletions(-)
> 
> CC-ing the rest of the maintainers (Intel ones, since Boris is
> on the AMD side).
> 
> I am OK with this patch going in Xen 4.5 as for one thing to actually
> use vpmu you have to pass 'vpmu=1' on the Xen command line.
> 
> Aka, Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>

> 
> >
> > Changes in v4:
> >  * Back to v2's IPI implementation
> >
> > Changes in v3:
> >  * Use cmpxchg instead of IPI
> >  * Use correct routine names 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..37f0d9f 100644
> > --- a/xen/arch/x86/hvm/vpmu.c
> > +++ b/xen/arch/x86/hvm/vpmu.c
> > @@ -247,10 +247,30 @@ void vpmu_initialise(struct vcpu *v)
> >      }
> >  }
> >
> > +static void vpmu_clear_last(void *arg)
> > +{
> > +    if ( this_cpu(last_vcpu) == arg )
> > +        this_cpu(last_vcpu) = NULL;
> > +}
> > +
> >  void vpmu_destroy(struct vcpu *v)
> >  {
> >      struct vpmu_struct *vpmu = vcpu_vpmu(v);
> >
> > +    if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_ALLOCATED) )
> > +        return;
> > +
> > +    /*
> > +     * Need to clear last_vcpu in case it points to v.
> > +     * We can check here non-atomically whether it is 'v' since
> > +     * last_vcpu can never become 'v' again at this point.
> > +     * We will test it again in vpmu_clear_last() with interrupts
> > +     * disabled to make sure we don't clear someone else.
> > +     */
> > +    if ( per_cpu(last_vcpu, vpmu->last_pcpu) == v )
> > +        on_selected_cpus(cpumask_of(vpmu->last_pcpu),
> > +                         vpmu_clear_last, v, 1);
> > +
> >      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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