[PATCH] x86/irq: Skip unmap_domain_pirq XSM during destruction

[Jason Andryuk <jandryuk@xxxxxxxxx>]

xsm_unmap_domain_irq was seen denying unmap_domain_pirq when called from
complete_domain_destroy as an RCU callback.  The source context was an
unexpected, random domain.  Since this is a xen-internal operation,
we don't want the XSM hook denying the operation.

Check d->is_dying and skip the check when the domain is dead.  The RCU
callback runs when a domain is in that state.

Suggested-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>
---
Dan wants to change current to point at DOMID_IDLE when the RCU callback
runs.  I think Juergen's commit 53594c7bd197 "rcu: don't use
stop_machine_run() for rcu_barrier()" may have changed this since it
mentions stop_machine_run scheduled the idle vcpus to run the callbacks
for the old code.

Would that be as easy as changing rcu_do_batch() to do:

+        /* Run as "Xen" not a random domain's vcpu. */
+        vcpu = get_current();
+        set_current(idle_vcpu[smp_processor_id()]);
         list->func(list);
+        set_current(vcpu);

or is using set_current() only acceptable as part of context_switch?

 xen/arch/x86/irq.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index 285ac399fb..16488d287c 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -2340,10 +2340,14 @@ int unmap_domain_pirq(struct domain *d, int pirq)
         nr = msi_desc->msi.nvec;
     }
 
-    ret = xsm_unmap_domain_irq(XSM_HOOK, d, irq,
-                               msi_desc ? msi_desc->dev : NULL);
-    if ( ret )
-        goto done;
+    /* When called by complete_domain_destroy via RCU, current is a random
+     * domain.  Skip the XSM check since this is a Xen-initiated action. */
+    if ( d->is_dying != DOMDYING_dead ) {
+        ret = xsm_unmap_domain_irq(XSM_HOOK, d, irq,
+                                   msi_desc ? msi_desc->dev : NULL);
+        if ( ret )
+            goto done;
+    }
 
     forced_unbind = pirq_guest_force_unbind(d, info);
     if ( forced_unbind )
-- 
2.35.1