Date:   Tue, 24 Dec 2019 13:22:46 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Andrew Murray <andrew.murray@....com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        <kvmarm@...ts.cs.columbia.edu>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 14/18] KVM: arm64: spe: Provide guest virtual  interrupts for SPE

On 2019-12-24 13:08, Andrew Murray wrote:
> On Tue, Dec 24, 2019 at 12:42:02PM +0000, Marc Zyngier wrote:
>> On 2019-12-24 11:50, Andrew Murray wrote:
>> > On Sun, Dec 22, 2019 at 12:07:50PM +0000, Marc Zyngier wrote:
>> > > On Fri, 20 Dec 2019 14:30:21 +0000,
>> > > Andrew Murray <andrew.murray@....com> wrote:
>> > > >
>> > > > Upon the exit of a guest, let's determine if the SPE device 
>> has
>> > > generated
>> > > > an interrupt - if so we'll inject a virtual interrupt to the
>> > > guest.
>> > > >
>> > > > Upon the entry and exit of a guest we'll also update the state 
>> of
>> > > the
>> > > > physical IRQ such that it is active when a guest interrupt is
>> > > pending
>> > > > and the guest is running.
>> > > >
>> > > > Finally we map the physical IRQ to the virtual IRQ such that 
>> the
>> > > guest
>> > > > can deactivate the interrupt when it handles the interrupt.
>> > > >
>> > > > Signed-off-by: Andrew Murray <andrew.murray@....com>
>> > > > ---
>> > > >  include/kvm/arm_spe.h |  6 ++++
>> > > >  virt/kvm/arm/arm.c    |  5 ++-
>> > > >  virt/kvm/arm/spe.c    | 71
>> > > +++++++++++++++++++++++++++++++++++++++++++
>> > > >  3 files changed, 81 insertions(+), 1 deletion(-)
>> > > >
>> > > > diff --git a/include/kvm/arm_spe.h b/include/kvm/arm_spe.h
>> > > > index 9c65130d726d..91b2214f543a 100644
>> > > > --- a/include/kvm/arm_spe.h
>> > > > +++ b/include/kvm/arm_spe.h
>> > > > @@ -37,6 +37,9 @@ static inline bool 
>> kvm_arm_support_spe_v1(void)
>> > > >  						      ID_AA64DFR0_PMSVER_SHIFT);
>> > > >  }
>> > > >
>> > > > +void kvm_spe_flush_hwstate(struct kvm_vcpu *vcpu);
>> > > > +inline void kvm_spe_sync_hwstate(struct kvm_vcpu *vcpu);
>> > > > +
>> > > >  int kvm_arm_spe_v1_set_attr(struct kvm_vcpu *vcpu,
>> > > >  			    struct kvm_device_attr *attr);
>> > > >  int kvm_arm_spe_v1_get_attr(struct kvm_vcpu *vcpu,
>> > > > @@ -49,6 +52,9 @@ int kvm_arm_spe_v1_enable(struct kvm_vcpu
>> > > *vcpu);
>> > > >  #define kvm_arm_support_spe_v1()	(false)
>> > > >  #define kvm_arm_spe_irq_initialized(v)	(false)
>> > > >
>> > > > +static inline void kvm_spe_flush_hwstate(struct kvm_vcpu 
>> *vcpu)
>> > > {}
>> > > > +static inline void kvm_spe_sync_hwstate(struct kvm_vcpu 
>> *vcpu) {}
>> > > > +
>> > > >  static inline int kvm_arm_spe_v1_set_attr(struct kvm_vcpu 
>> *vcpu,
>> > > >  					  struct kvm_device_attr *attr)
>> > > >  {
>> > > > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>> > > > index 340d2388ee2c..a66085c8e785 100644
>> > > > --- a/virt/kvm/arm/arm.c
>> > > > +++ b/virt/kvm/arm/arm.c
>> > > > @@ -741,6 +741,7 @@ int kvm_arch_vcpu_ioctl_run(struct 
>> kvm_vcpu
>> > > *vcpu, struct kvm_run *run)
>> > > >  		preempt_disable();
>> > > >
>> > > >  		kvm_pmu_flush_hwstate(vcpu);
>> > > > +		kvm_spe_flush_hwstate(vcpu);
>> > > >
>> > > >  		local_irq_disable();
>> > > >
>> > > > @@ -782,6 +783,7 @@ int kvm_arch_vcpu_ioctl_run(struct 
>> kvm_vcpu
>> > > *vcpu, struct kvm_run *run)
>> > > >  		    kvm_request_pending(vcpu)) {
>> > > >  			vcpu->mode = OUTSIDE_GUEST_MODE;
>> > > >  			isb(); /* Ensure work in x_flush_hwstate is committed */
>> > > > +			kvm_spe_sync_hwstate(vcpu);
>> > > >  			kvm_pmu_sync_hwstate(vcpu);
>> > > >  			if (static_branch_unlikely(&userspace_irqchip_in_use))
>> > > >  				kvm_timer_sync_hwstate(vcpu);
>> > > > @@ -816,11 +818,12 @@ int kvm_arch_vcpu_ioctl_run(struct 
>> kvm_vcpu
>> > > *vcpu, struct kvm_run *run)
>> > > >  		kvm_arm_clear_debug(vcpu);
>> > > >
>> > > >  		/*
>> > > > -		 * We must sync the PMU state before the vgic state so
>> > > > +		 * We must sync the PMU and SPE state before the vgic state 
>> so
>> > > >  		 * that the vgic can properly sample the updated state of 
>> the
>> > > >  		 * interrupt line.
>> > > >  		 */
>> > > >  		kvm_pmu_sync_hwstate(vcpu);
>> > > > +		kvm_spe_sync_hwstate(vcpu);
>> > >
>> > > The *HUGE* difference is that the PMU is purely a virtual 
>> interrupt,
>> > > while you're trying to deal with a HW interrupt here.
>> > >
>> > > >
>> > > >  		/*
>> > > >  		 * Sync the vgic state before syncing the timer state 
>> because
>> > > > diff --git a/virt/kvm/arm/spe.c b/virt/kvm/arm/spe.c
>> > > > index 83ac2cce2cc3..097ed39014e4 100644
>> > > > --- a/virt/kvm/arm/spe.c
>> > > > +++ b/virt/kvm/arm/spe.c
>> > > > @@ -35,6 +35,68 @@ int kvm_arm_spe_v1_enable(struct kvm_vcpu
>> > > *vcpu)
>> > > >  	return 0;
>> > > >  }
>> > > >
>> > > > +static inline void set_spe_irq_phys_active(struct
>> > > arm_spe_kvm_info *info,
>> > > > +					   bool active)
>> > > > +{
>> > > > +	int r;
>> > > > +	r = irq_set_irqchip_state(info->physical_irq,
>> > > IRQCHIP_STATE_ACTIVE,
>> > > > +				  active);
>> > > > +	WARN_ON(r);
>> > > > +}
>> > > > +
>> > > > +void kvm_spe_flush_hwstate(struct kvm_vcpu *vcpu)
>> > > > +{
>> > > > +	struct kvm_spe *spe = &vcpu->arch.spe;
>> > > > +	bool phys_active = false;
>> > > > +	struct arm_spe_kvm_info *info = arm_spe_get_kvm_info();
>> > > > +
>> > > > +	if (!kvm_arm_spe_v1_ready(vcpu))
>> > > > +		return;
>> > > > +
>> > > > +	if (irqchip_in_kernel(vcpu->kvm))
>> > > > +		phys_active = kvm_vgic_map_is_active(vcpu, spe->irq_num);
>> > > > +
>> > > > +	phys_active |= spe->irq_level;
>> > > > +
>> > > > +	set_spe_irq_phys_active(info, phys_active);
>> > >
>> > > So you're happy to mess with the HW interrupt state even when 
>> you
>> > > don't have a HW irqchip? If you are going to copy paste the 
>> timer
>> > > code
>> > > here, you'd need to support it all the way (no, don't).
>> > >
>> > > > +}
>> > > > +
>> > > > +void kvm_spe_sync_hwstate(struct kvm_vcpu *vcpu)
>> > > > +{
>> > > > +	struct kvm_spe *spe = &vcpu->arch.spe;
>> > > > +	u64 pmbsr;
>> > > > +	int r;
>> > > > +	bool service;
>> > > > +	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
>> > > > +	struct arm_spe_kvm_info *info = arm_spe_get_kvm_info();
>> > > > +
>> > > > +	if (!kvm_arm_spe_v1_ready(vcpu))
>> > > > +		return;
>> > > > +
>> > > > +	set_spe_irq_phys_active(info, false);
>> > > > +
>> > > > +	pmbsr = ctxt->sys_regs[PMBSR_EL1];
>> > > > +	service = !!(pmbsr & BIT(SYS_PMBSR_EL1_S_SHIFT));
>> > > > +	if (spe->irq_level == service)
>> > > > +		return;
>> > > > +
>> > > > +	spe->irq_level = service;
>> > > > +
>> > > > +	if (likely(irqchip_in_kernel(vcpu->kvm))) {
>> > > > +		r = kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
>> > > > +					spe->irq_num, service, spe);
>> > > > +		WARN_ON(r);
>> > > > +	}
>> > > > +}
>> > > > +
>> > > > +static inline bool kvm_arch_arm_spe_v1_get_input_level(int
>> > > vintid)
>> > > > +{
>> > > > +	struct kvm_vcpu *vcpu = kvm_arm_get_running_vcpu();
>> > > > +	struct kvm_spe *spe = &vcpu->arch.spe;
>> > > > +
>> > > > +	return spe->irq_level;
>> > > > +}
>> > >
>> > > This isn't what such a callback is for. It is supposed to sample 
>> the
>> > > HW, an nothing else.
>> > >
>> > > > +
>> > > >  static int kvm_arm_spe_v1_init(struct kvm_vcpu *vcpu)
>> > > >  {
>> > > >  	if (!kvm_arm_support_spe_v1())
>> > > > @@ -48,6 +110,7 @@ static int kvm_arm_spe_v1_init(struct 
>> kvm_vcpu
>> > > *vcpu)
>> > > >
>> > > >  	if (irqchip_in_kernel(vcpu->kvm)) {
>> > > >  		int ret;
>> > > > +		struct arm_spe_kvm_info *info;
>> > > >
>> > > >  		/*
>> > > >  		 * If using the SPE with an in-kernel virtual GIC
>> > > > @@ -57,10 +120,18 @@ static int kvm_arm_spe_v1_init(struct
>> > > kvm_vcpu *vcpu)
>> > > >  		if (!vgic_initialized(vcpu->kvm))
>> > > >  			return -ENODEV;
>> > > >
>> > > > +		info = arm_spe_get_kvm_info();
>> > > > +		if (!info->physical_irq)
>> > > > +			return -ENODEV;
>> > > > +
>> > > >  		ret = kvm_vgic_set_owner(vcpu, vcpu->arch.spe.irq_num,
>> > > >  					 &vcpu->arch.spe);
>> > > >  		if (ret)
>> > > >  			return ret;
>> > > > +
>> > > > +		ret = kvm_vgic_map_phys_irq(vcpu, info->physical_irq,
>> > > > +					    vcpu->arch.spe.irq_num,
>> > > > +					    kvm_arch_arm_spe_v1_get_input_level);
>> > >
>> > > You're mapping the interrupt int the guest, and yet you have 
>> never
>> > > forwarded the interrupt the first place. All this flow is only 
>> going
>> > > to wreck the host driver as soon as an interrupt occurs.
>> > >
>> > > I think you should rethink the interrupt handling altogether. It
>> > > would
>> > > make more sense if the interrupt was actually completely
>> > > virtualized. If you can isolate the guest state and compute the
>> > > interrupt state in SW (and from the above, it seems that you 
>> can),
>> > > then you shouldn't mess with the whole forwarding *at all*, as 
>> it
>> > > isn't designed for devices shared between host and guests.
>> >
>> > Yes it's possible to read SYS_PMBSR_EL1_S_SHIFT and determine if 
>> SPE
>> > wants
>> > service. If I understand correctly, you're suggesting on 
>> entry/exit to
>> > the
>> > guest we determine this and inject an interrupt to the guest. As 
>> well as
>> > removing the kvm_vgic_map_phys_irq mapping to the physical 
>> interrupt?
>>
>> The mapping only makes sense for devices that have their interrupt
>> forwarded to a vcpu, where the expected flow is that the interrupt
>> is taken on the host with a normal interrupt handler and then
>> injected in the guest (you still have to manage the active state
>> though). The basic assumption is that such a device is entirely
>> owned by KVM.
>
> Though the mapping does mean that if the guest handles the guest SPE
> interrupt it doesn't have to wait for a guest exit before having the
> SPE interrupt evaluated again (i.e. another SPE interrupt won't cause
> a guest exit) - thus increasing the size of any black hole.

Sure. It still remains that your use case is outside of the scope of
this internal API.

>> Here, you're abusing the mapping interface: you don't have an
>> interrupt handler (the host SPE driver owns it), the interrupt
>> isn't forwarded, and yet you're messing with the active state.
>> None of that is expected, and you are in uncharted territory
>> as far as KVM is concerned.
>>
>> What bothers me the most is that this looks a lot like a previous
>> implementation of the timers, and we had all the problems in the
>> world to keep track of the interrupt state *and* have a reasonable
>> level of performance (hitting the redistributor on the fast path
>> is a performance killer).
>>
>> > My understanding was that I needed knowledge of the physical SPE
>> > interrupt
>> > number so that I could prevent the host SPE driver from getting 
>> spurious
>> > interrupts due to guest use of the SPE.
>>
>> You can't completely rule out the host getting interrupted. Even if 
>> you set
>> PMBSR_EL1.S to zero, there is no guarantee that the host will not 
>> observe
>> the interrupt anyway (the GIC architecture doesn't tell you how 
>> quickly
>> it will be retired, if ever). The host driver already checks for 
>> this
>> anyway.
>>
>> What you need to ensure is that PMBSR_EL1.S being set on guest entry
>> doesn't immediately kick you out of the guest and prevent forward
>> progress. This is why you need to manage the active state.
>>
>> The real question is: how quickly do you want to react to a SPE
>> interrupt firing while in a guest?
>>
>> If you want to take it into account as soon as it fires, then you 
>> need
>> to eagerly save/restore the active state together with the SPE state 
>> on
>> each entry/exit, and performance will suffer. This is what you are
>> currently doing.
>>
>> If you're OK with evaluating the interrupt status on exit, but 
>> without
>> the interrupt itself causing an exit, then you can simply manage it
>> as a purely virtual interrupt, and just deal with the active state
>> in load/put (set the interrupt as active on load, clear it on put).
>
> This does feel like the pragmatic approach - a larger black hole in 
> exchange
> for performance. I imagine the blackhole would be naturally reduced 
> on
> machines with high workloads.

Why? I don't see the relation between how busy the vcpu is and the size
of the blackhole. It is strictly a function of the frequency of exits.

         M.

>
> I'll refine the series to take this approach.
>
>>
>> Given that SPE interrupts always indicate that profiling has 
>> stopped,
>
> and faults :|
>
> Thanks,
>
> Andrew Murray
>
>> this only affects the size of the black hole, and I'm inclined to do
>> the latter.
>>
>>         M.
>> --
>> Jazz is not dead. It just smells funny...

-- 
Jazz is not dead. It just smells funny...

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