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

From: [email-masked] (Marc Zyngier)
Date: Mon, 15 Dec 2014 08:57:58 +0000

On 14/12/14 14:14, Christoffer Dall wrote:
> 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 ?

Seems like a sensible idea. You could even make the "enable" part of
kvm_timer_vcpu_init(), and move that call to the "first run" section. Up
to you, really.

Thanks,

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