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Re: [Xen-devel] [RFC PATCH v2 21/22] ARM: vITS: injecting LPIs: use pending_irq lock

Hi Andre,

On 21/07/17 21:00, Andre Przywara wrote:
Instead of using an atomic access and hoping for the best, let's use
the new pending_irq lock now to make sure we read a sane version of
the target VCPU.

How this is going to bring a saner version?

You only read the vCPU and well nothing prevent it to change between the time you get it and lock it in vgic_vcpu_inject_irq.


That still doesn't solve the problem mentioned in the comment, but
paves the way for future improvements.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
 xen/arch/arm/gic-v3-lpi.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/xen/arch/arm/gic-v3-lpi.c b/xen/arch/arm/gic-v3-lpi.c
index 2306b58..9db26ed 100644
--- a/xen/arch/arm/gic-v3-lpi.c
+++ b/xen/arch/arm/gic-v3-lpi.c
@@ -140,20 +140,22 @@ void vgic_vcpu_inject_lpi(struct domain *d, unsigned int 
      * TODO: this assumes that the struct pending_irq stays valid all of
-     * the time. We cannot properly protect this with the current locking
-     * scheme, but the future per-IRQ lock will solve this problem.
+     * the time. We cannot properly protect this with the current code,
+     * but a future refcounting will solve this problem.
     struct pending_irq *p = irq_to_pending(d->vcpu[0], virq);
+    unsigned long flags;
     unsigned int vcpu_id;

     if ( !p )

-    vcpu_id = ACCESS_ONCE(p->vcpu_id);
-    if ( vcpu_id >= d->max_vcpus )
-          return;
+    vgic_irq_lock(p, flags);
+    vcpu_id = p->vcpu_id;
+    vgic_irq_unlock(p, flags);

-    vgic_vcpu_inject_irq(d->vcpu[vcpu_id], virq);
+    if ( vcpu_id < d->max_vcpus )
+        vgic_vcpu_inject_irq(d->vcpu[vcpu_id], virq);


Julien Grall

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