x86/VPMU: Clear last_vcpu when destroying VPMU

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@oracle.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
master commit: ed80171556
master date: 2015-01-07 11:12:27 +0100

xen/arch/x86/hvm/svm/vpmu.c

@@ -398,9 +398,6 @@ 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);
 

xen/arch/x86/hvm/vmx/vpmu_core2.c

@@ -783,8 +783,6 @@ 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 )

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);
 }