Re: [PATCHv0 dont apply] RFC: kvm eoi PV using shared memory

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Apr 10, 2012 at 05:03:22PM +0300, Avi Kivity wrote:
> On 04/10/2012 04:27 PM, Michael S. Tsirkin wrote:
> > I took a stub at implementing PV EOI using shared memory.
> > This should reduce the number of exits an interrupt
> > causes as much as by half.
> >
> > A partially complete draft for both host and guest parts
> > is below.
> >
> > The idea is simple: there's a bit, per APIC, in guest memory,
> > that tells the guest that it does not need EOI.
> > We set it before injecting an interrupt and clear
> > before injecting a nested one. Guest tests it using
> > a test and clear operation - this is necessary
> > so that host can detect interrupt nesting -
> > and if set, it can skip the EOI MSR.
> >
> > There's a new MSR to set the address of said register
> > in guest memory. Otherwise not much changes:
> > - Guest EOI is not required
> > - ISR is automatically cleared before injection
> >
> > Some things are incomplete: add feature negotiation
> > options, qemu support for said options.
> > No testing was done beyond compiling the kernel.
> >
> > I would appreciate early feedback.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> >
> > --
> >
> > diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
> > index d854101..8430f41 100644
> > --- a/arch/x86/include/asm/apic.h
> > +++ b/arch/x86/include/asm/apic.h
> > @@ -457,8 +457,13 @@ static inline u32 safe_apic_wait_icr_idle(void) { return 0; }
> >  
> >  #endif /* CONFIG_X86_LOCAL_APIC */
> >  
> > +DECLARE_EARLY_PER_CPU(unsigned long, apic_eoi);
> > +
> >  static inline void ack_APIC_irq(void)
> >  {
> > +	if (__test_and_clear_bit(0, &__get_cpu_var(apic_eoi)))
> > +		return;
> > +
> 
> While __test_and_clear_bit() is implemented in a single instruction,
> it's not required to be.  Better have the instruction there explicitly.
> 
> >  	/*
> >  	 * ack_APIC_irq() actually gets compiled as a single instruction
> >  	 * ... yummie.
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index e216ba0..0ee1472 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -481,6 +481,12 @@ struct kvm_vcpu_arch {
> >  		u64 length;
> >  		u64 status;
> >  	} osvw;
> > +
> > +	struct {
> > +		u64 msr_val;
> > +		struct gfn_to_hva_cache data;
> > +		int vector;
> > +	} eoi;
> >  };
> 
> Needs to be cleared on INIT.

You mean kvm_arch_vcpu_reset?

> >  
> >
> > @@ -307,6 +308,9 @@ void __cpuinit kvm_guest_cpu_init(void)
> >  		       smp_processor_id());
> >  	}
> >  
> > +	wrmsrl(MSR_KVM_EOI_EN, __pa(this_cpu_ptr(apic_eoi)) |
> > +	       MSR_KVM_EOI_ENABLED);
> > +
> 
> Clear on kexec.

With register_reboot_notifier?

> >  	if (has_steal_clock)
> >  		kvm_register_steal_time();
> >  }
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 8584322..9e38e12 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -265,7 +265,61 @@ int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq)
> >  			irq->level, irq->trig_mode);
> >  }
> >  
> > -static inline int apic_find_highest_isr(struct kvm_lapic *apic)
> > +static int eoi_put_user(struct kvm_vcpu *vcpu, u32 val)
> > +{
> > +
> > +	return kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.eoi.data, &val,
> > +				      sizeof(val));
> > +}
> > +
> > +static int eoi_get_user(struct kvm_vcpu *vcpu, u32 *val)
> > +{
> > +
> > +	return kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.eoi.data, val,
> > +				      sizeof(*val));
> > +}
> > +
> > +static inline bool eoi_enabled(struct kvm_vcpu *vcpu)
> > +{
> > +	return (vcpu->arch.eoi.msr_val & MSR_KVM_EOI_ENABLED);
> > +}
> > +
> > +static int eoi_get_pending_vector(struct kvm_vcpu *vcpu)
> > +{
> > +	u32 val;
> > +	if (eoi_get_user(vcpu, &val) < 0)
> > +		apic_debug("Can't read EOI MSR value: 0x%llx\n",
> > +			   (unsigned long long)vcpi->arch.eoi.msr_val);
> > +	if (!(val & 0x1))
> > +		vcpu->arch.eoi.vector = -1;
> > +	return vcpu->arch.eoi.vector;
> > +}
> > +
> > +static void eoi_set_pending_vector(struct kvm_vcpu *vcpu, int vector)
> > +{
> > +	BUG_ON(vcpu->arch.eoi.vector != -1);
> > +	if (eoi_put_user(vcpu, 0x1) < 0) {
> > +		apic_debug("Can't set EOI MSR value: 0x%llx\n",
> > +			   (unsigned long long)vcpi->arch.eoi.msr_val);
> > +		return;
> > +	}
> > +	vcpu->arch.eoi.vector = vector;
> > +}
> > +
> > +static int eoi_clr_pending_vector(struct kvm_vcpu *vcpu)
> > +{
> > +	int vector;
> > +	vector = vcpu->arch.eoi.vector;
> > +	if (vector != -1 && eoi_put_user(vcpu, 0x0) < 0) {
> > +		apic_debug("Can't clear EOI MSR value: 0x%llx\n",
> > +			   (unsigned long long)vcpi->arch.eoi.msr_val);
> > +		return -1;
> > +	}
> > +	vcpu->arch.eoi.vector = -1;
> > +	return vector;
> > +}
> 
> 
> 
> > +
> > +static inline int __apic_find_highest_isr(struct kvm_lapic *apic)
> >  {
> >  	int result;
> >  
> > @@ -275,6 +329,17 @@ static inline int apic_find_highest_isr(struct kvm_lapic *apic)
> >  	return result;
> >  }
> >  
> > +static inline int apic_find_highest_isr(struct kvm_lapic *apic)
> > +{
> > +	int vector;
> > +	if (eoi_enabled(apic->vcpu)) {
> > +		vector = eoi_get_pending_vector(apic->vcpu);
> > +		if (vector != -1)
> > +			return vector;
> > +	}
> > +	return __apic_find_highest_isr(apic);
> > +}
> 
> Why aren't you modifying the ISR unconfitionally?

ISR is not set if there won't be an EOI
since it's EOI that normally clears it.

> > +
> >  static void apic_update_ppr(struct kvm_lapic *apic)
> >  {
> >  	u32 tpr, isrv, ppr, old_ppr;
> > @@ -488,6 +553,8 @@ static void apic_set_eoi(struct kvm_lapic *apic)
> >  	if (vector == -1)
> >  		return;
> >  
> > +	if (eoi_enabled(apic->vcpu))
> > +		eoi_clr_pending_vector(apic->vcpu);
> >  	apic_clear_vector(vector, apic->regs + APIC_ISR);
> >  	apic_update_ppr(apic);
> >  
> > @@ -1236,11 +1303,25 @@ int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu)
> >  {
> >  	int vector = kvm_apic_has_interrupt(vcpu);
> >  	struct kvm_lapic *apic = vcpu->arch.apic;
> > +	bool set_isr = true;
> >  
> >  	if (vector == -1)
> >  		return -1;
> >  
> > -	apic_set_vector(vector, apic->regs + APIC_ISR);
> > +	if (eoi_enabled(vcpu)) {
> > +		/* Anything pending? If yes disable eoi optimization. */
> > +		if (unlikely(apic_find_highest_isr(apic) >= 0)) {
> > +			int v = eoi_clr_pending_vector(vcpu);
> 
> ISR != pending, that's IRR.  If ISR vector has lower priority than the
> new vector, then we don't need to disable eoi avoidance.

Yes. But we can and it's easier than figuring out priorities.
I am guessing such collisions are rare, right?
I'll add a trace to make sure.

> > +			if (v != -1)
> > +				apic_set_vector(v, apic->regs + APIC_ISR);
> > +		} else {
> > +			eoi_set_pending_vector(vcpu, vector);
> > +			set_isr = false;
> 
> Weird.  Just set it normally.  Remember that reading the ISR needs to
> return the correct value.

Marcelo said linux does not normally read ISR - not true?
Note this has no effect if the PV optimization is not enabled.

> We need to process the avoided EOI before any APIC read/writes, to be
> sure the guest sees the updated values.  Same for IOAPIC, EOI affects
> remote_irr.  That may been we need to sample it after every exit, or
> perhaps disable the feature for level-triggered interrupts.

Disabling would be very sad.  Can we sample on remote irr read?

> > +		}
> > +	}
> > +
> > +	if (set_isr)
> > +		apic_set_vector(vector, apic->regs + APIC_ISR);
> >  	apic_update_ppr(apic);
> >  	apic_clear_irr(vector, apic);
> >  	return vector;
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >
> 
> -- 
> error compiling committee.c: too many arguments to function