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Date:   Mon, 30 Jan 2017 15:45:50 +0100
From:   Christoffer Dall <christoffer.dall@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Jintack Lim <jintack@...columbia.edu>, pbonzini@...hat.com,
        rkrcmar@...hat.com, linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, andre.przywara@....com, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 02/10] KVM: arm/arm64: Move cntvoff to each timer context

On Sun, Jan 29, 2017 at 11:54:05AM +0000, Marc Zyngier wrote:
> On Fri, Jan 27 2017 at 01:04:52 AM, Jintack Lim <jintack@...columbia.edu> wrote:
> > Make cntvoff per each timer context. This is helpful to abstract kvm
> > timer functions to work with timer context without considering timer
> > types (e.g. physical timer or virtual timer).
> >
> > This also would pave the way for ever doing adjustments of the cntvoff
> > on a per-CPU basis if that should ever make sense.
> >
> > Signed-off-by: Jintack Lim <jintack@...columbia.edu>
> > ---
> >  arch/arm/include/asm/kvm_host.h   |  6 +++---
> >  arch/arm64/include/asm/kvm_host.h |  4 ++--
> >  include/kvm/arm_arch_timer.h      |  8 +++-----
> >  virt/kvm/arm/arch_timer.c         | 26 ++++++++++++++++++++------
> >  virt/kvm/arm/hyp/timer-sr.c       |  3 +--
> >  5 files changed, 29 insertions(+), 18 deletions(-)
> >
> > diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> > index d5423ab..f5456a9 100644
> > --- a/arch/arm/include/asm/kvm_host.h
> > +++ b/arch/arm/include/asm/kvm_host.h
> > @@ -60,9 +60,6 @@ struct kvm_arch {
> >  	/* The last vcpu id that ran on each physical CPU */
> >  	int __percpu *last_vcpu_ran;
> >  
> > -	/* Timer */
> > -	struct arch_timer_kvm	timer;
> > -
> >  	/*
> >  	 * Anything that is not used directly from assembly code goes
> >  	 * here.
> > @@ -75,6 +72,9 @@ struct kvm_arch {
> >  	/* Stage-2 page table */
> >  	pgd_t *pgd;
> >  
> > +	/* A lock to synchronize cntvoff among all vtimer context of vcpus */
> > +	spinlock_t cntvoff_lock;
> 
> Is there any condition where we need this to be a spinlock? I would have
> thought that a mutex should have been enough, as this should only be
> updated on migration or initialization. Not that it matters much in this
> case, but I wondered if there is something I'm missing.
> 

I would think the critical section is small enough that a spinlock makes
sense, but what I don't think we need is to add the additional lock.

I think just taking the kvm->lock should be sufficient, which happens to
be a mutex, and while that may be a bit slower to take than the
spinlock, it's not in the critical path so let's just keep things
simple.

Perhaps this what Marc also meant.

> > +
> >  	/* Interrupt controller */
> >  	struct vgic_dist	vgic;
> >  	int max_vcpus;
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index e505038..23749a8 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -71,8 +71,8 @@ struct kvm_arch {
> >  	/* Interrupt controller */
> >  	struct vgic_dist	vgic;
> >  
> > -	/* Timer */
> > -	struct arch_timer_kvm	timer;
> > +	/* A lock to synchronize cntvoff among all vtimer context of vcpus */
> > +	spinlock_t cntvoff_lock;
> >  };
> >  
> >  #define KVM_NR_MEM_OBJS     40
> > diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
> > index daad3c1..1b9c988 100644
> > --- a/include/kvm/arm_arch_timer.h
> > +++ b/include/kvm/arm_arch_timer.h
> > @@ -23,11 +23,6 @@
> >  #include <linux/hrtimer.h>
> >  #include <linux/workqueue.h>
> >  
> > -struct arch_timer_kvm {
> > -	/* Virtual offset */
> > -	u64			cntvoff;
> > -};
> > -
> >  struct arch_timer_context {
> >  	/* Registers: control register, timer value */
> >  	u32				cnt_ctl;
> > @@ -38,6 +33,9 @@ struct arch_timer_context {
> >  
> >  	/* Active IRQ state caching */
> >  	bool				active_cleared_last;
> > +
> > +	/* Virtual offset */
> > +	u64			cntvoff;
> >  };
> >  
> >  struct arch_timer_cpu {
> > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > index 6740efa..fa4c042 100644
> > --- a/virt/kvm/arm/arch_timer.c
> > +++ b/virt/kvm/arm/arch_timer.c
> > @@ -101,9 +101,10 @@ static void kvm_timer_inject_irq_work(struct work_struct *work)
> >  static u64 kvm_timer_compute_delta(struct kvm_vcpu *vcpu)
> >  {
> >  	u64 cval, now;
> > +	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> >  
> > -	cval = vcpu_vtimer(vcpu)->cnt_cval;
> > -	now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
> > +	cval = vtimer->cnt_cval;
> > +	now = kvm_phys_timer_read() - vtimer->cntvoff;
> >  
> >  	if (now < cval) {
> >  		u64 ns;
> > @@ -159,7 +160,7 @@ bool kvm_timer_should_fire(struct kvm_vcpu *vcpu)
> >  		return false;
> >  
> >  	cval = vtimer->cnt_cval;
> > -	now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
> > +	now = kvm_phys_timer_read() - vtimer->cntvoff;
> >  
> >  	return cval <= now;
> >  }
> > @@ -353,10 +354,23 @@ int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
> >  	return 0;
> >  }
> >  
> > +/* Make the updates of cntvoff for all vtimer contexts atomic */
> > +static void update_vtimer_cntvoff(struct kvm_vcpu *vcpu, u64 cntvoff)
> 
> Arguably, this acts on the VM itself and not a single vcpu. maybe you
> should consider passing the struct kvm pointer to reflect this.
> 
> > +{
> > +	int i;
> > +
> > +	spin_lock(&vcpu->kvm->arch.cntvoff_lock);
> > +	kvm_for_each_vcpu(i, vcpu, vcpu->kvm)
> > +		vcpu_vtimer(vcpu)->cntvoff = cntvoff;
> > +	spin_unlock(&vcpu->kvm->arch.cntvoff_lock);
> > +}
> > +
> >  void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu)
> >  {
> >  	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> >  
> > +	update_vtimer_cntvoff(vcpu, kvm_phys_timer_read());
> 
> Maybe a comment indicating that we recompute CNTVOFF for all vcpus would
> be welcome (this is not a change in semantics, but it was never obvious
> in the existing code).
> 
> > +
> >  	INIT_WORK(&timer->expired, kvm_timer_inject_irq_work);
> >  	hrtimer_init(&timer->timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
> >  	timer->timer.function = kvm_timer_expire;
> > @@ -376,7 +390,7 @@ int kvm_arm_timer_set_reg(struct kvm_vcpu *vcpu, u64 regid, u64 value)
> >  		vtimer->cnt_ctl = value;
> >  		break;
> >  	case KVM_REG_ARM_TIMER_CNT:
> > -		vcpu->kvm->arch.timer.cntvoff = kvm_phys_timer_read() - value;
> > +		update_vtimer_cntvoff(vcpu, kvm_phys_timer_read() - value);
> >  		break;
> >  	case KVM_REG_ARM_TIMER_CVAL:
> >  		vtimer->cnt_cval = value;
> > @@ -397,7 +411,7 @@ u64 kvm_arm_timer_get_reg(struct kvm_vcpu *vcpu, u64 regid)
> >  	case KVM_REG_ARM_TIMER_CTL:
> >  		return vtimer->cnt_ctl;
> >  	case KVM_REG_ARM_TIMER_CNT:
> > -		return kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
> > +		return kvm_phys_timer_read() - vtimer->cntvoff;
> >  	case KVM_REG_ARM_TIMER_CVAL:
> >  		return vtimer->cnt_cval;
> >  	}
> > @@ -511,7 +525,7 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
> >  
> >  void kvm_timer_init(struct kvm *kvm)
> >  {
> > -	kvm->arch.timer.cntvoff = kvm_phys_timer_read();
> > +	spin_lock_init(&kvm->arch.cntvoff_lock);
> >  }
> >  
> >  /*
> > diff --git a/virt/kvm/arm/hyp/timer-sr.c b/virt/kvm/arm/hyp/timer-sr.c
> > index 0cf0895..4734915 100644
> > --- a/virt/kvm/arm/hyp/timer-sr.c
> > +++ b/virt/kvm/arm/hyp/timer-sr.c
> > @@ -53,7 +53,6 @@ void __hyp_text __timer_save_state(struct kvm_vcpu *vcpu)
> >  
> >  void __hyp_text __timer_restore_state(struct kvm_vcpu *vcpu)
> >  {
> > -	struct kvm *kvm = kern_hyp_va(vcpu->kvm);
> >  	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> >  	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> >  	u64 val;
> > @@ -71,7 +70,7 @@ void __hyp_text __timer_restore_state(struct kvm_vcpu *vcpu)
> >  	}
> >  
> >  	if (timer->enabled) {
> > -		write_sysreg(kvm->arch.timer.cntvoff, cntvoff_el2);
> > +		write_sysreg(vtimer->cntvoff, cntvoff_el2);
> >  		write_sysreg_el0(vtimer->cnt_cval, cntv_cval);
> >  		isb();
> >  		write_sysreg_el0(vtimer->cnt_ctl, cntv_ctl);
> 
I agree with the other two comments as well.

Otherwise looks ok.

Thanks,
-Christoffer

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