[Xen-changelog] [xen stable-4.4] x86/VPMU: Clear last_vcpu when destroying VPMU

[patchbot@xxxxxxx]

commit 2dcdc46d789316baf7126c17f5d39b60153c1bed
Author:     Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
AuthorDate: Tue Feb 3 12:28:40 2015 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Feb 3 12:28:40 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>
    master commit: ed8017155607db1bbe1f6ca41eac696b7ef8082b
    master date: 2015-01-07 11:12:27 +0100
---
 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 66a3815..d7ab815 100644
--- a/xen/arch/x86/hvm/svm/vpmu.c
+++ b/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);
 
diff --git a/xen/arch/x86/hvm/vmx/vpmu_core2.c 
b/xen/arch/x86/hvm/vmx/vpmu_core2.c
index f6e8ec0..833d710 100644
--- a/xen/arch/x86/hvm/vmx/vpmu_core2.c
+++ b/xen/arch/x86/hvm/vmx/vpmu_core2.c
@@ -809,8 +809,6 @@ 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 21fbaba..27441e2 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#stable-4.4

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