Re: [PATCH v3 6/6] arm/arm64: KVM: Require in-kernel vgic for the arch timers

[Marc Zyngier <marc.zyngier@arm.com>]

On 15/12/14 10:20, Christoffer Dall 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 <christoffer.dall@linaro.org>
> ---
> Changes [v2 -> v3]:
>  - Split kvm_timer_init into kvm_timer_init and kvm_timer_enable
>    and initialize the cntvoff in kvm_timer_init and only actually enable
>    the timer if there is an in-kernel vgic.
>  - Added comment about race from multiple VCPUs.
>  - Support compiling on 32-bit ARM wihtout vgic/arch-timers config
>    option.
> 
>  arch/arm/kvm/arm.c           | 13 +++++++++++--
>  include/kvm/arm_arch_timer.h | 10 ++++------
>  virt/kvm/arm/arch_timer.c    | 30 ++++++++++++++++++++++--------
>  3 files changed, 37 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index d4da244..8cadfec 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -424,6 +424,7 @@ 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 +436,20 @@ 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;
>  	}
>  
> +	/*
> +	 * Enable the arch 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 gic.
> +	 */
> +	if (irqchip_in_kernel(kvm) && vgic_initialized(kvm))
> +		kvm_timer_enable(kvm);
> +
>  	return 0;
>  }
>  
> diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
> index ad9db60..b3f45a5 100644
> --- a/include/kvm/arm_arch_timer.h
> +++ b/include/kvm/arm_arch_timer.h
> @@ -60,7 +60,8 @@ 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_enable(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);
> @@ -77,11 +78,8 @@ static inline int kvm_timer_hyp_init(void)
>  	return 0;
>  };
>  
> -static inline int kvm_timer_init(struct kvm *kvm)
> -{
> -	return 0;
> -}
> -
> +static inline void kvm_timer_enable(struct kvm *kvm) {}
> +static inline void kvm_timer_init(struct kvm *kvm) {}
>  static inline void kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
>  					const struct kvm_irq_level *irq) {}
>  static inline 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..1c0772b 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 +309,24 @@ void kvm_timer_vcpu_terminate(struct kvm_vcpu *vcpu)
>  	timer_disarm(timer);
>  }
>  
> -int kvm_timer_init(struct kvm *kvm)
> +void kvm_timer_enable(struct kvm *kvm)
>  {
> -	if (timecounter && wqueue) {
> -		kvm->arch.timer.cntvoff = kvm_phys_timer_read();
> +	if (kvm->arch.timer.enabled)
> +		return;
> +
> +	/*
> +	 * There is a potential race here between VCPUs starting for the first
> +	 * time, which may be enabling the timer multiple times.  That doesn't
> +	 * hurt though, because we're just setting a variable to the same
> +	 * variable that it already was.  The important thing is that all
> +	 * VCPUs have the enabled variable set, before entering the guest, if
> +	 * the arch timers are enabled.
> +	 */
> +	if (timecounter && wqueue)
>  		kvm->arch.timer.enabled = 1;
> -	}
> +}

This is particularly interesting, as this paves the way for per-vcpu
enable bits, meaning that we won't have to extract the enable bit from
struct kvm while doing a world switch. Clearly a fight for another day
though.

>  
> -	return 0;
> +void kvm_timer_init(struct kvm *kvm)
> +{
> +	kvm->arch.timer.cntvoff = kvm_phys_timer_read();
>  }
> 

Looks good to me.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>

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