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Re: [Xen-devel] [RFC PATCH 09/49] ARM: VGIC: change to level-IRQ compatible IRQ injection interface

On 12/02/18 11:59, Andre Przywara wrote:

Hi Andre,

On 12/02/18 11:15, Julien Grall wrote:
Hi Andre,

On 09/02/18 14:38, Andre Przywara wrote:
diff --git a/xen/arch/arm/vgic.c b/xen/arch/arm/vgic.c
index 5f47aa84a9..2fc6e19625 100644
--- a/xen/arch/arm/vgic.c
+++ b/xen/arch/arm/vgic.c
@@ -285,7 +285,7 @@ bool vgic_migrate_irq(struct vcpu *old, struct
vcpu *new, unsigned int irq)
           vgic_remove_irq_from_queues(old, p);
           irq_set_affinity(p->desc, cpumask_of(new->processor));
           spin_unlock_irqrestore(&old->arch.vgic.lock, flags);
-        vgic_vcpu_inject_irq(new, irq);
+        vgic_inject_irq(new->domain, new, irq, true);
           return true;
       /* if the IRQ is in a GICH_LR register, set GIC_IRQ_GUEST_MIGRATING
@@ -444,7 +444,7 @@ bool vgic_to_sgi(struct vcpu *v, register_t sgir,
enum gic_sgi_mode irqmode,
                           sgir, target->list);
-            vgic_vcpu_inject_irq(d->vcpu[vcpuid], virq);
+            vgic_inject_irq(d, d->vcpu[vcpuid], virq, true);
       case SGI_TARGET_OTHERS:
@@ -453,12 +453,12 @@ bool vgic_to_sgi(struct vcpu *v, register_t
sgir, enum gic_sgi_mode irqmode,
               if ( i != current->vcpu_id && d->vcpu[i] != NULL &&
                    is_vcpu_online(d->vcpu[i]) )
-                vgic_vcpu_inject_irq(d->vcpu[i], virq);
+                vgic_inject_irq(d, d->vcpu[i], virq, true);
       case SGI_TARGET_SELF:
-        vgic_vcpu_inject_irq(d->vcpu[current->vcpu_id], virq);
+        vgic_inject_irq(d, current, virq, true);
@@ -518,13 +518,29 @@ void vgic_remove_irq_from_queues(struct vcpu *v,
struct pending_irq *p)
       gic_remove_from_lr_pending(v, p);
   -void vgic_vcpu_inject_irq(struct vcpu *v, unsigned int virq)
+int vgic_inject_irq(struct domain *d, struct vcpu *v, unsigned int virq,
+                    bool level)

Looking at the code after the series has been applied, no one is caring
about the return value of vgic_inject_irq. So what is the rationale
behind changing the return type from void to int?

The KVM version returns an error value, in particular when:
- the VGIC has not been initialized yet
- we can't determine the VCPU for a private interrupt
- the interrupt ID is invalid (SPI beyond limit, not mapped LPI)
In the moment it's not very useful for Xen: the first two conditions
don't really happen, consequently I removed those checks. But the third
check may become interesting once we get LPIs. Also since Xen currently
uses a void prototype for injection, *this* patch *now* doesn't exploit
the newly gained possibility of properly handling errors. From briefly
checking all the users, all of them seem to be in void functions, so
indeed an error return is not very useful.
The reasons I kept it in was to allow introduction of checks later. I
think having a function returning an error where some users ignore this
is better than the other way round.

I don't think it is much better. This is a way to expose yet another security issue because the return is not correctly checked (see XSA-246 for instance). Any return value should be checked or have a comment explaining why it is fine.

So of course I can easily make this void, but I wonder what we do in
those cases where the SPI is not valid, for instance? Shall we print
some (rate-limited) warning?

I can understand why KVM needs such interface as the interrupt controller may be emulated QEMU. But I can't see why a SPI would not be valid in Xen context (except programming error). So could give an example?

What would you expect the caller to do on error? Except printing an error message?


Julien Grall

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