linux-kernel - Re: [PATCHv2] kvm: optimize ISR lookups

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Date:	Thu, 31 May 2012 13:06:57 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] kvm: optimize ISR lookups

On Wed, May 30, 2012 at 05:54:42PM -0300, Marcelo Tosatti wrote:
> On Tue, May 22, 2012 at 03:54:56PM +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. 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 might also benefit.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> > 
> > I am well aware of Thomas and Peter's suggestion of reworking APIC
> > register handling in kvm instead of adding a cache like this patch does.
> > 
> > This revision does *not* address that comment yet: it only corrects a
> > bug in the original patch.
> > 
> > Posting in this form for ease of testing.
> > 
> > Changes from v1:
> > 	replace ASSERT by BUG_ON, correcting inverted logic
> > 
> >  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..0d2985d 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;
> > +	BUG_ON(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;
> > +	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 +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))
> 
> assert(isr_count == 1).
> 
> Looks fine otherwise. Gleb can you review please?
I am not convinced we need to keep track of isr_count. isr_cache should be enough.
Why setting isr_cache to -1 if isr_count > 1? Just overwrite isr_cache
values with newly injected vector and use it on next EOI.

--
			Gleb.
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