[PATCH] KVM: arm/arm64: Fix arch timers with userspace irqchips

[Marc Zyngier]

On 30/01/18 12:46, Christoffer Dall wrote:
> When introducing support for irqchip in userspace we needed a way to
> mask the timer signal to prevent the guest continuously exiting due to a
> screaming timer.
> 
> We did this by disabling the corresponding percpu interrupt on the
> host interrupt controller, because we cannot rely on the host system
> having a GIC, and therefore cannot make any assumptions about having an
> active state to hide the timer signal.
> 
> Unfortunately, when introducing this feature, it became entirely
> possible that a VCPU which belongs to a VM that has a userspace irqchip
> can disable the vtimer irq on the host on some physical CPU, and then go
> away without ever enabling the vimter irq on that physical CPU again.

vtimer

> 
> This means that using irqchips in userspace on a system that also
> supports running VMs with an in-kernel GIC can prevent forward progress
> from in-kernel GIC VMs.
> 
> Later on, when we started taking virtual timer interrupts in the arch
> timer code, we would also leave this timer state active for userspace
> irqchip VMs, because we leave it up to a VGIC-enabled guest to
> deactivate the hardware IRQ using the HW bit in the LR.
> 
> Both issues are solved by only using the enable/disable trick on systems
> that do not have a host GIC which supports the active state, because all
> VMs on such systems must use irqchips in userspace.  Systems that have a
> working GIC with support for an active state use the active state to
> mask the timer signal for both userspace an in-kernel irqchips.
> 
> Cc: Alexander Graf <agraf at suse.de>
> Cc: <stable at vger.kernel.org> # v4.12+
> Fixes: d9e139778376 ("KVM: arm/arm64: Support arch timers with a userspace gic")
> Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
> ---
> This conflicts horribly with everything when applied to either
> kvmarm/queue or kvmarm/master.  Therefore, this patch is written for
> (and applies to) v4.15 with kvmarm/queue merged and should therefore
> apply cleanly after v4.16-rc1.  An example with this patch applied can
> be found on kvmarm/temp-for-v4.16-rc2.  I plan on sending this along
> with any other potential fixes post v4.16-rc1.
> 
>  virt/kvm/arm/arch_timer.c | 77 ++++++++++++++++++++++++++---------------------
>  1 file changed, 42 insertions(+), 35 deletions(-)
> 
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 70268c0bec79..228906ceb722 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -35,6 +35,7 @@
>  static struct timecounter *timecounter;
>  static unsigned int host_vtimer_irq;
>  static u32 host_vtimer_irq_flags;
> +static bool has_gic_active_state;
>  
>  static const struct kvm_irq_level default_ptimer_irq = {
>  	.irq	= 30,
> @@ -69,25 +70,6 @@ static void soft_timer_cancel(struct hrtimer *hrt, struct work_struct *work)
>  		cancel_work_sync(work);
>  }
>  
> -static void kvm_vtimer_update_mask_user(struct kvm_vcpu *vcpu)
> -{
> -	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> -
> -	/*
> -	 * When using a userspace irqchip with the architected timers, we must
> -	 * prevent continuously exiting from the guest, and therefore mask the
> -	 * physical interrupt by disabling it on the host interrupt controller
> -	 * when the virtual level is high, such that the guest can make
> -	 * forward progress.  Once we detect the output level being
> -	 * de-asserted, we unmask the interrupt again so that we exit from the
> -	 * guest when the timer fires.
> -	 */
> -	if (vtimer->irq.level)
> -		disable_percpu_irq(host_vtimer_irq);
> -	else
> -		enable_percpu_irq(host_vtimer_irq, 0);
> -}
> -
>  static irqreturn_t kvm_arch_timer_handler(int irq, void *dev_id)
>  {
>  	struct kvm_vcpu *vcpu = *(struct kvm_vcpu **)dev_id;
> @@ -107,8 +89,8 @@ static irqreturn_t kvm_arch_timer_handler(int irq, void *dev_id)
>  		kvm_timer_update_irq(vcpu, true, vtimer);
>  
>  	if (static_branch_unlikely(&userspace_irqchip_in_use) &&
> -	    unlikely(!irqchip_in_kernel(vcpu->kvm)))
> -		kvm_vtimer_update_mask_user(vcpu);
> +	    unlikely(!irqchip_in_kernel(vcpu->kvm)) && !has_gic_active_state)
> +		disable_percpu_irq(host_vtimer_irq);

nit: this is the negated version of the fix you posted earlier
(0e23cb34af26 in kvmarm/queue), plus a new term... How about rewriting
this as:

static inline bool userspace_irqchip(struct kvm *kvm)
{
	return (static_branch_unlikely(&userspace_irqchip_in_use) &&
		unlikely(!irqchip_in_kernel(kvm)));
}

and this becomes:

	if (userspace_irqchip(vcpu->kvm) && !has_gic_active_state)
		[...]

which I find much more readable.

Same for kvm_timer_update_irq():

	if (!userspace_irqchip(vcpu->kvm))
		[...]

>  
>  	return IRQ_HANDLED;
>  }
> @@ -460,13 +442,16 @@ static void set_cntvoff(u64 cntvoff)
>  	kvm_call_hyp(__kvm_timer_set_cntvoff, low, high);
>  }
>  
> -static void kvm_timer_vcpu_load_vgic(struct kvm_vcpu *vcpu)
> +static void kvm_timer_vcpu_load_gic(struct kvm_vcpu *vcpu)
>  {
>  	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
>  	bool phys_active;
>  	int ret;
>  
> -	phys_active = kvm_vgic_map_is_active(vcpu, vtimer->irq.irq);
> +	if (irqchip_in_kernel(vcpu->kvm))
> +		phys_active = kvm_vgic_map_is_active(vcpu, vtimer->irq.irq);
> +	else
> +		phys_active = vtimer->irq.level;

You could use the above helper here too.

>  
>  	ret = irq_set_irqchip_state(host_vtimer_irq,
>  				    IRQCHIP_STATE_ACTIVE,
> @@ -474,9 +459,24 @@ static void kvm_timer_vcpu_load_vgic(struct kvm_vcpu *vcpu)
>  	WARN_ON(ret);
>  }
>  
> -static void kvm_timer_vcpu_load_user(struct kvm_vcpu *vcpu)
> +static void kvm_timer_vcpu_load_nogic(struct kvm_vcpu *vcpu)
>  {
> -	kvm_vtimer_update_mask_user(vcpu);
> +	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> +
> +	/*
> +	 * When using a userspace irqchip with the architected timers and a
> +	 * host interrupt controller that doesn't support an active state, we
> +	 * must still we must prevent continuously exiting from the guest, and
> +	 * therefore mask the physical interrupt by disabling it on the host
> +	 * interrupt controller when the virtual level is high, such that the
> +	 * guest can make forward progress.  Once we detect the output level
> +	 * being de-asserted, we unmask the interrupt again so that we exit
> +	 * from the guest when the timer fires.
> +	 */
> +	if (vtimer->irq.level)
> +		disable_percpu_irq(host_vtimer_irq);
> +	else
> +		enable_percpu_irq(host_vtimer_irq, host_vtimer_irq_flags);
>  }
>  
>  void kvm_timer_vcpu_load(struct kvm_vcpu *vcpu)
> @@ -487,10 +487,10 @@ void kvm_timer_vcpu_load(struct kvm_vcpu *vcpu)
>  	if (unlikely(!timer->enabled))
>  		return;
>  
> -	if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
> -		kvm_timer_vcpu_load_user(vcpu);
> +	if (has_gic_active_state)

likely()?

> +		kvm_timer_vcpu_load_gic(vcpu);
>  	else
> -		kvm_timer_vcpu_load_vgic(vcpu);
> +		kvm_timer_vcpu_load_nogic(vcpu);
>  
>  	set_cntvoff(vtimer->cntvoff);
>  
> @@ -555,18 +555,23 @@ static void unmask_vtimer_irq_user(struct kvm_vcpu *vcpu)
>  {
>  	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
>  
> -	if (unlikely(!irqchip_in_kernel(vcpu->kvm))) {
> -		__timer_snapshot_state(vtimer);
> -		if (!kvm_timer_should_fire(vtimer)) {
> -			kvm_timer_update_irq(vcpu, false, vtimer);
> -			kvm_vtimer_update_mask_user(vcpu);
> -		}
> +	__timer_snapshot_state(vtimer);
> +	if (!kvm_timer_should_fire(vtimer)) {
> +		kvm_timer_update_irq(vcpu, false, vtimer);
> +		if (!has_gic_active_state)

unlikely()?

> +			enable_percpu_irq(host_vtimer_irq, host_vtimer_irq_flags);
>  	}
>  }
>  
>  void kvm_timer_sync_hwstate(struct kvm_vcpu *vcpu)
>  {
> -	unmask_vtimer_irq_user(vcpu);
> +	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> +
> +	if (unlikely(!timer->enabled))
> +		return;
> +
> +	if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
> +		unmask_vtimer_irq_user(vcpu);
>  }
>  
>  int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu)
> @@ -753,6 +758,8 @@ int kvm_timer_hyp_init(bool has_gic)
>  			kvm_err("kvm_arch_timer: error setting vcpu affinity\n");
>  			goto out_free_irq;
>  		}
> +
> +		has_gic_active_state = true;

or even better, how about turning this into a static key? This is a
one-off thing, and nobody is going to remove the GIC from the system, so
we might as well optimize it in our favour.

>  	}
>  
>  	kvm_info("virtual timer IRQ%d\n", host_vtimer_irq);
> 

These nits aside, this looks like the right thing to do.

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

Thanks,

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