Re: [PATCHv6 2/8] kvm: optimize ISR lookups

Marcelo Tosatti
2012-06-12T21:08:33Z

On Sun, Jun 03, 2012 at 10:27:59AM +0300, Michael S. Tsirkin wrote:
> 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. on set, caching the injected vector number
> 3. on clear, 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: with this patch,
> and without interrupt nesting, apic_find_highest_isr
> will always return immediately without scanning ISR.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>
> This revision is slightly reworked from the last version
> I sent: I don't invalidate the cache when there is more
> than one bit set in ISR. This makes the code simpler,
> makes cache valid in more cases and avoids a branch on data path.
>
> Otherwise this is basically the same as v2: this revision does not
> yet address Thomas's idea of reworking the APIC page handling.
>
> arch/x86/kvm/lapic.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
> arch/x86/kvm/lapic.h | 4 ++++
> 2 files changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 93c1574..db54e63 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 +262,22 @@ 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;
> + BUG_ON(apic->isr_count > MAX_APIC_VECTOR);
> + apic->isr_cache = vec;
> +}
> +
> +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;
> + BUG_ON(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 +309,10 @@ 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;

What about TPR updates that modify highest ISR available? They are not
fully covered. Ex: writes to TPR register not via CR8.

apic_update_ppr should reset cache?

Instead of isr_cache what about a highest_isr field?

When setting ISR:

if (apic->highest_isr < me)
apic->highest_isr = me;

To be invalidated on TPR updates properly.

Its more meaningful.