Re: [RFC v2 02/10] KVM: arm/arm64: Move cntvoff to each timer context

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

On Fri, Jan 27 2017 at 01:04:52 AM, Jintack Lim <jintack@cs.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@cs.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.

> +
>  	/* 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);

Thanks,

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