[PATCH] kvm: optimize ISR lookups

Michael S. Tsirkin
2012-05-21T16:46:11Z

We perform ISR lookups twice: during interrupt
injection and on EOI. Typical workloads only have
a single bit set there. So we can avoid ISR scans by
1. counting bits as we set/clear them in ISR
2. if count is 1, caching the vector number
3. if count != 1, invalidating the cache

The real purpose of this is enabling PV EOI
which needs to quickly validate the vector.
But non PV guests also benefit.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
arch/x86/kvm/lapic.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++-
arch/x86/kvm/lapic.h | 2 +
2 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 93c1574..232950a 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -107,6 +107,16 @@ static inline void apic_clear_vector(int vec, void *bitmap)
clear_bit(VEC_POS(vec), (bitmap) + REG_POS(vec));
}

+static inline int __apic_test_and_set_vector(int vec, void *bitmap)
+{
+ return __test_and_set_bit(VEC_POS(vec), (bitmap) + REG_POS(vec));
+}
+
+static inline int __apic_test_and_clear_vector(int vec, void *bitmap)
+{
+ return __test_and_clear_bit(VEC_POS(vec), (bitmap) + REG_POS(vec));
+}
+
static inline int apic_hw_enabled(struct kvm_lapic *apic)
{
return (apic)->vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE;
@@ -210,6 +220,16 @@ static int find_highest_vector(void *bitmap)
return fls(word[word_offset << 2]) - 1 + (word_offset << 5);
}

+static u8 count_vectors(void *bitmap)
+{
+ u32 *word = bitmap;
+ int word_offset;
+ u8 count = 0;
+ for (word_offset = 0; word_offset < MAX_APIC_VECTOR >> 5; ++word_offset)
+ count += hweight32(word[word_offset << 2]);
+ return count;
+}
+
static inline int apic_test_and_set_irr(int vec, struct kvm_lapic *apic)
{
apic->irr_pending = true;
@@ -242,6 +262,25 @@ static inline void apic_clear_irr(int vec, struct kvm_lapic *apic)
apic->irr_pending = true;
}

+static inline void apic_set_isr(int vec, struct kvm_lapic *apic)
+{
+ if (!__apic_test_and_set_vector(vec, apic->regs + APIC_ISR))
+ ++apic->isr_count;
+ ASSERT(apic->isr_count > MAX_APIC_VECTOR);
+ if (likely(apic->isr_count == 1))
+ apic->isr_cache = vec;
+ else
+ apic->isr_cache = -1;
+}
+
+static inline void apic_clear_isr(int vec, struct kvm_lapic *apic)
+{
+ if (__apic_test_and_clear_vector(vec, apic->regs + APIC_ISR))
+ --apic->isr_count;
+ ASSERT(apic->isr_count < 0);
+ apic->isr_cache = -1;
+}
+
int kvm_lapic_find_highest_irr(struct kvm_vcpu *vcpu)
{
struct kvm_lapic *apic = vcpu->arch.apic;
@@ -273,6 +312,10 @@ int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq)
static inline int apic_find_highest_isr(struct kvm_lapic *apic)
{
int result;
+ if (!apic->isr_count)
+ return -1;
+ if (likely(apic->isr_cache != -1))
+ return apic->isr_cache;

result = find_highest_vector(apic->regs + APIC_ISR);
ASSERT(result == -1 || result >= 16);
@@ -492,7 +535,7 @@ static void apic_set_eoi(struct kvm_lapic *apic)
if (vector == -1)
return;

- apic_clear_vector(vector, apic->regs + APIC_ISR);
+ apic_clear_isr(vector, apic);
apic_update_ppr(apic);

if (!(apic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI) &&
@@ -1081,6 +1124,8 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu)
apic_set_reg(apic, APIC_TMR + 0x10 * i, 0);
}
apic->irr_pending = false;
+ apic->isr_count = 0;
+ apic->isr_cache = -1;
update_divide_count(apic);
atomic_set(&apic->lapic_timer.pending, 0);
if (kvm_vcpu_is_bsp(vcpu))
@@ -1248,7 +1293,7 @@ int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu)
if (vector == -1)
return -1;

- apic_set_vector(vector, apic->regs + APIC_ISR);
+ apic_set_isr(vector, apic);
apic_update_ppr(apic);
apic_clear_irr(vector, apic);
return vector;
@@ -1267,6 +1312,8 @@ void kvm_apic_post_state_restore(struct kvm_vcpu *vcpu)
update_divide_count(apic);
start_apic_timer(apic);
apic->irr_pending = true;
+ apic->isr_count = count_vectors(apic->regs + APIC_ISR);
+ apic->isr_cache = -1;
kvm_make_request(KVM_REQ_EVENT, vcpu);
}

diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 6f4ce25..9f8deff 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -13,6 +13,8 @@ struct kvm_lapic {
u32 divide_count;
struct kvm_vcpu *vcpu;
bool irr_pending;
+ s16 isr_count;
+ int isr_cache;
void *regs;
gpa_t vapic_addr;
struct page *vapic_page;
--
MST