[Xen-changelog] [xen master] x86/VPMU: Clear last_vcpu when destroying VPMU

[patchbot@xxxxxxx]

commit ed8017155607db1bbe1f6ca41eac696b7ef8082b
Author:     Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
AuthorDate: Wed Jan 7 11:12:27 2015 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jan 7 11:12:27 2015 +0100

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

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);
 }
--
generated by git-patchbot for /home/xen/git/xen.git#master

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