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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Dec 17, 2014 at 11:53:41AM -0500, Boris Ostrovsky wrote:
> 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?

Never hurts. I would just respond to this email with the patch as attachment
and inline for easier commit.
> 
> -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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