[PATCH v2 3/5] x86/hvm: fix ISA IRQ 0 handling when set as lowest priority mode in IO APIC

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Lowest priority destination mode does allow the vIO APIC code to
select a vCPU to inject the interrupt to, but the selected vCPU must
be part of the possible destinations configured for such IO APIC pin.

Fix the code in order to only force vCPU 0 if it's part of the
listed destinations.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Add a comment regarding the vlapic_enabled check.
---
NB: I haven't added a fallback to vCPU 0 if no destination is found,
as it's not how real hardware behaves. I think we should assume that
no user have relied on this bogus Xen behavior for IRQ 0 interrupt
injection.
---
 xen/arch/x86/hvm/vioapic.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/hvm/vioapic.c b/xen/arch/x86/hvm/vioapic.c
index 123191db75..67d4a6237f 100644
--- a/xen/arch/x86/hvm/vioapic.c
+++ b/xen/arch/x86/hvm/vioapic.c
@@ -415,12 +415,14 @@ static void vioapic_deliver(struct hvm_vioapic *vioapic, 
unsigned int pin)
     case dest_LowestPrio:
     {
 #ifdef IRQ0_SPECIAL_ROUTING
-        /* Force round-robin to pick VCPU 0 */
-        if ( (irq == hvm_isa_irq_to_gsi(0)) && pt_active(&d->arch.vpit.pt0) )
-        {
-            v = d->vcpu ? d->vcpu[0] : NULL;
-            target = v ? vcpu_vlapic(v) : NULL;
-        }
+        struct vlapic *lapic0 = vcpu_vlapic(d->vcpu[0]);
+
+        /* Force to pick vCPU 0 if part of the destination list */
+        if ( (irq == hvm_isa_irq_to_gsi(0)) && pt_active(&d->arch.vpit.pt0) &&
+             vlapic_match_dest(lapic0, NULL, 0, dest, dest_mode) &&
+             /* Mimic the vlapic_enabled check found in vlapic_lowest_prio. */
+             vlapic_enabled(lapic0) )
+            target = lapic0;
         else
 #endif
             target = vlapic_lowest_prio(d, NULL, 0, dest, dest_mode);
-- 
2.27.0