x86, hvm: Implement interrupt routing to least priority processor.

Instead of round robin the vcpu with the lowest processor
priority is selected for the interrupt. If multiple vcpus
share the same low priority then interrupts are distributed between
those round robin.

Signed-off-by: Juergen Gross <juergen.gross@fujitsu-siemens.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

1 files changed, 20 insertions, 16 deletions

diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c
index d201af2848..68e9b27632 100644
--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -377,26 +377,30 @@ static int vlapic_accept_irq(struct vcpu *v, int delivery_mode,
 }
 
 /* This function is used by both ioapic and lapic.The bitmap is for vcpu_id. */
-struct vlapic *apic_round_robin(
-    struct domain *d, uint8_t vector, uint32_t bitmap)
+struct vlapic *apic_lowest_prio(struct domain *d, uint32_t bitmap)
 {
-    int next, old;
-    struct vlapic *target = NULL;
+    int old = d->arch.hvm_domain.irq.round_robin_prev_vcpu;
+    uint32_t ppr, target_ppr = UINT_MAX;
+    struct vlapic *vlapic, *target = NULL;
+    struct vcpu *v;
 
-    old = next = d->arch.hvm_domain.irq.round_robin_prev_vcpu;
+    if ( unlikely((v = d->vcpu[old]) == NULL) )
+        return NULL;
 
     do {
-        if ( ++next == MAX_VIRT_CPUS ) 
-            next = 0;
-        if ( (d->vcpu[next] == NULL) || !test_bit(next, &bitmap) )
-            continue;
-        target = vcpu_vlapic(d->vcpu[next]);
-        if ( vlapic_enabled(target) )
-            break;
-        target = NULL;
-    } while ( next != old );
+        v = v->next_in_list ? : d->vcpu[0];
+        vlapic = vcpu_vlapic(v);
+        if ( test_bit(v->vcpu_id, &bitmap) && vlapic_enabled(vlapic) &&
+             ((ppr = vlapic_get_ppr(vlapic)) < target_ppr) )
+        {
+            target = vlapic;
+            target_ppr = ppr;
+        }
+    } while ( v->vcpu_id != old );
 
-    d->arch.hvm_domain.irq.round_robin_prev_vcpu = next;
+    if ( target != NULL )
+        d->arch.hvm_domain.irq.round_robin_prev_vcpu =
+            vlapic_vcpu(target)->vcpu_id;
 
     return target;
 }
@@ -456,7 +460,7 @@ int vlapic_ipi(
 
     if ( delivery_mode == APIC_DM_LOWEST )
     {
-        target = apic_round_robin(vlapic_domain(v), vector, lpr_map);
+        target = apic_lowest_prio(vlapic_domain(v), lpr_map);
         if ( target != NULL )
             rc = vlapic_accept_irq(vlapic_vcpu(target), delivery_mode,
                                    vector, level, trig_mode);