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

[Marcelo Tosatti <mtosatti@redhat.com>]

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.