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>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index b1694dc..3bde43c 100644 (file)
--- 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))

diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index a587f83..3f3ecc6 100644 (file)
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -12,6 +12,7 @@ struct kvm_lapic {
        struct kvm_timer lapic_timer;
        u32 divide_count;
        struct kvm_vcpu *vcpu;
+       bool irr_pending;
        struct page *regs_page;
        void *regs;
        gpa_t vapic_addr;