Subject: [PATCH v2 6/6] arm/arm64: KVM: Require in-kernel vgic for the arch timers

From: [email-masked] (Christoffer Dall)
Date: Sun, 14 Dec 2014 15:14:01 +0100

On Sun, Dec 14, 2014 at 11:33:04AM +0000, Marc Zyngier wrote:
> On Sat, Dec 13 2014 at 11:17:29 AM, Christoffer Dall <[email-masked]> wrote:
> > It is curently possible to run a VM with architected timers support
> > without creating an in-kernel VGIC, which will result in interrupts from
> > the virtual timer going nowhere.
> >
> > To address this issue, move the architected timers initialization to the
> > time when we run a VCPU for the first time, and then only initialize
> > (and enable) the architected timers if we have a properly created and
> > initialized in-kernel VGIC.
> >
> > When injecting interrupts from the virtual timer to the vgic, the
> > current setup should ensure that this never calls an on-demand init of
> > the VGIC, which is the only call path that could return an error from
> > kvm_vgic_inject_irq(), so capture the return value and raise a warning
> > if there's an error there.
> >
> > We also change the kvm_timer_init() function from returning an int to be
> > a void function, since the function always succeeds.
> >
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  arch/arm/kvm/arm.c           | 15 +++++++++++----
> >  include/kvm/arm_arch_timer.h |  2 +-
> >  virt/kvm/arm/arch_timer.c    | 15 ++++++++++-----
> >  3 files changed, 22 insertions(+), 10 deletions(-)
> >
> > diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> > index d4da244..06f0431 100644
> > --- a/arch/arm/kvm/arm.c
> > +++ b/arch/arm/kvm/arm.c
> > @@ -127,8 +127,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >  	if (ret)
> >  		goto out_free_stage2_pgd;
> >  
> > -	kvm_timer_init(kvm);
> > -
> >  	/* Mark the initial VMID generation invalid */
> >  	kvm->arch.vmid_gen = 0;
> >  
> > @@ -424,6 +422,7 @@ static void update_vttbr(struct kvm *kvm)
> >  
> >  static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
> >  {
> > +	struct kvm *kvm = vcpu->kvm;
> >  	int ret;
> >  
> >  	if (likely(vcpu->arch.has_run_once))
> > @@ -435,12 +434,20 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
> >  	 * Map the VGIC hardware resources before running a vcpu the first
> >  	 * time on this VM.
> >  	 */
> > -	if (unlikely(!vgic_ready(vcpu->kvm))) {
> > -		ret = kvm_vgic_map_resources(vcpu->kvm);
> > +	if (unlikely(!vgic_ready(kvm))) {
> > +		ret = kvm_vgic_map_resources(kvm);
> >  		if (ret)
> >  			return ret;
> >  	}
> >  
> > +	/*
> > +	 * Initialize the Architected Timers only if we have an in-kernel VGIC
> > +	 * and it has been properly initialized, since we cannot handle
> > +	 * interrupts from the virtual timer with a userspace vgic.
> > +	 */
> > +	if (irqchip_in_kernel(kvm) && vgic_initialized(kvm))
> > +		kvm_timer_init(kvm);
> > +
> >  	return 0;
> >  }
> >  
> > diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
> > index ad9db60..c9bd045 100644
> > --- a/include/kvm/arm_arch_timer.h
> > +++ b/include/kvm/arm_arch_timer.h
> > @@ -60,7 +60,7 @@ struct arch_timer_cpu {
> >  
> >  #ifdef CONFIG_KVM_ARM_TIMER
> >  int kvm_timer_hyp_init(void);
> > -int kvm_timer_init(struct kvm *kvm);
> > +void kvm_timer_init(struct kvm *kvm);
> >  void kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
> >  			  const struct kvm_irq_level *irq);
> >  void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu);
> > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > index 22fa819..48ce5cb 100644
> > --- a/virt/kvm/arm/arch_timer.c
> > +++ b/virt/kvm/arm/arch_timer.c
> > @@ -61,12 +61,14 @@ static void timer_disarm(struct arch_timer_cpu *timer)
> >  
> >  static void kvm_timer_inject_irq(struct kvm_vcpu *vcpu)
> >  {
> > +	int ret;
> >  	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> >  
> >  	timer->cntv_ctl |= ARCH_TIMER_CTRL_IT_MASK;
> > -	kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
> > -			    timer->irq->irq,
> > -			    timer->irq->level);
> > +	ret = kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
> > +				  timer->irq->irq,
> > +				  timer->irq->level);
> > +	WARN_ON(ret);
> >  }
> >  
> >  static irqreturn_t kvm_arch_timer_handler(int irq, void *dev_id)
> > @@ -307,12 +309,15 @@ void kvm_timer_vcpu_terminate(struct kvm_vcpu *vcpu)
> >  	timer_disarm(timer);
> >  }
> >  
> > -int kvm_timer_init(struct kvm *kvm)
> > +void kvm_timer_init(struct kvm *kvm)
> >  {
> > +	if (kvm->arch.timer.enabled)
> > +		return;
> > +
> >  	if (timecounter && wqueue) {
> >  		kvm->arch.timer.cntvoff = kvm_phys_timer_read();
> >  		kvm->arch.timer.enabled = 1;
> >  	}
> 
> Be careful, you've now introduced a race between two vcpus doing their
> "first run" at the same time. The consequence is fairly minor (only the
> virtual offset is affected, and that's unlikely to cause any ill effect
> that early in the life of the VM), but still.
> 
> We can decide that this is not big enough a deal to warrant a lock, but
> that definitely deserves a comment.

That escaped my attention completely, thanks for spotting it.

> 
> Another thing to consider is how this works with restoring a VM. We
> relied on the fact that CNTVOFF is set by system register accesses after
> the timer init, but we're now overriding the value. Am I missing
> something crucial?
> 

The answer to how it works is: It doesn't.

So the cleanest approach would be to initialize the cntvoff when
creating the VM, then enabling it at first vcpu run, adding a comment
saying that enabling the timer multiple times doesn't hurt.

Consequently adding a kvm_timer_enable() function that gets called on
first vcpu_run ?

> >  
> > -	return 0;
> > +	return;
> >  }
> 
> Don't bother with the return ;-).
> 
Right, pattern matching is my thing. ;)

-Christoffer