KVM: Optimize searching for highest IRR

Most of the time IRR is empty, so instead of scanning the whole IRR on
each VM entry keep a variable that tells us if IRR is not empty. IRR
will have to be scanned twice on each IRQ delivery, but this is much
more rare than VM entry.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 21 insertions, 3 deletions

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index b1694dc45729..3bde43c3789e 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -165,29 +165,46 @@ static int find_highest_vector(void *bitmap)
 
 static inline int apic_test_and_set_irr(int vec, struct kvm_lapic *apic)
 {
+        apic->irr_pending = true;
         return apic_test_and_set_vector(vec, apic->regs + APIC_IRR);
 }
 
-static inline void apic_clear_irr(int vec, struct kvm_lapic *apic)
+static inline int apic_search_irr(struct kvm_lapic *apic)
 {
-        apic_clear_vector(vec, apic->regs + APIC_IRR);
+        return find_highest_vector(apic->regs + APIC_IRR);
 }
 
 static inline int apic_find_highest_irr(struct kvm_lapic *apic)
 {
         int result;
 
-        result = find_highest_vector(apic->regs + APIC_IRR);
+        if (!apic->irr_pending)
+                return -1;
+
+        result = apic_search_irr(apic);
         ASSERT(result == -1 || result >= 16);
 
         return result;
 }
 
+static inline void apic_clear_irr(int vec, struct kvm_lapic *apic)
+{
+        apic->irr_pending = false;
+        apic_clear_vector(vec, apic->regs + APIC_IRR);
+        if (apic_search_irr(apic) != -1)
+                apic->irr_pending = true;
+}
+
 int kvm_lapic_find_highest_irr(struct kvm_vcpu *vcpu)
 {
         struct kvm_lapic *apic = vcpu->arch.apic;
         int highest_irr;
 
+        /* This may race with setting of irr in __apic_accept_irq() and
+         * value returned may be wrong, but kvm_vcpu_kick() in __apic_accept_irq
+         * will cause vmexit immediately and the value will be recalculated
+         * on the next vmentry.
+         */
         if (!apic)
                 return 0;
         highest_irr = apic_find_highest_irr(apic);
@@ -843,6 +860,7 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu)
                 apic_set_reg(apic, APIC_ISR + 0x10 * i, 0);
                 apic_set_reg(apic, APIC_TMR + 0x10 * i, 0);
         }
+        apic->irr_pending = false;
         update_divide_count(apic);
         atomic_set(&apic->lapic_timer.pending, 0);
         if (kvm_vcpu_is_bsp(vcpu))