[PATCH v3 3/5] KVM: arm/arm64: Support arch timers with a userspace gic

[marc.zyngier@arm.com (Marc Zyngier)]

On 06/04/17 09:16, Alexander Graf wrote:
> 
> 
> On 05.04.17 11:28, Christoffer Dall wrote:
>> From: Alexander Graf <agraf@suse.de>
>>
>> If you're running with a userspace gic or other interrupt constroller
>> (that is no vgic in the kernel), then you have so far not been able to
>> use the architected timers, because the output of the architected
>> timers, which are driven inside the kernel, was a kernel-only construct
>> between the arch timer code and the vgic.
>>
>> This patch implements the new KVM_CAP_ARM_USER_IRQ feature, where we use a
>> side channel on the kvm_run structure, run->s.regs.device_irq_level, to
>> always notify userspace of the timer output levels when using a userspace
>> irqchip.
>>
>> This works by ensureing that before we enter the guest, if the timer
>> output level has changed compared to what we last told userspace, we
>> don't enter the guest, but instead return to userspace to notify it of
>> the new level.  If we are exiting, because of an MMIO for example, and
>> the level changed at the same time, the value is also updated and
>> userspace can sample the line as it needs.  This is nicely achieved
>> simply always updating the timer_irq_level field after the main run
>> loop.
>>
>> Note that the kvm_timer_update_irq trace event is changed to show the
>> host IRQ number for the timer instead of the guest IRQ number, because
>> the kernel no longer know which IRQ userspace wires up the timer signal
>> to.
>>
>> Also note that this patch implements all required functionality but does
>> not yet advertise the capability.
>>
>> Signed-off-by: Alexander Graf <agraf@suse.de>
>> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
>> ---
>>  arch/arm/kvm/arm.c           |  18 +++----
>>  include/kvm/arm_arch_timer.h |   2 +
>>  virt/kvm/arm/arch_timer.c    | 122 +++++++++++++++++++++++++++++++++++--------
>>  3 files changed, 110 insertions(+), 32 deletions(-)
>>
>> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
>> index 7fa4898..efb16e5 100644
>> --- a/arch/arm/kvm/arm.c
>> +++ b/arch/arm/kvm/arm.c
>> @@ -515,13 +515,7 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>>  			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))
>> -		ret = kvm_timer_enable(vcpu);
>> +	ret = kvm_timer_enable(vcpu);
>>
>>  	return ret;
>>  }
>> @@ -640,9 +634,12 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>  		local_irq_disable();
>>
>>  		/*
>> -		 * Re-check atomic conditions
>> +		 * If we have a singal pending, or need to notify a userspace
>> +		 * irqchip about timer level changes, then we exit (and update
>> +		 * the timer level state in kvm_timer_update_run below).
>>  		 */
>> -		if (signal_pending(current)) {
>> +		if (signal_pending(current) ||
>> +		    kvm_timer_should_notify_user(vcpu)) {
>>  			ret = -EINTR;
>>  			run->exit_reason = KVM_EXIT_INTR;
>>  		}
>> @@ -714,6 +711,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>  		ret = handle_exit(vcpu, run, ret);
>>  	}
>>
>> +	/* Tell userspace about in-kernel device output levels */
>> +	kvm_timer_update_run(vcpu);
>> +
>>  	if (vcpu->sigset_active)
>>  		sigprocmask(SIG_SETMASK, &sigsaved, NULL);
>>  	return ret;
>> diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
>> index fe797d6..295584f 100644
>> --- a/include/kvm/arm_arch_timer.h
>> +++ b/include/kvm/arm_arch_timer.h
>> @@ -63,6 +63,8 @@ int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
>>  void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu);
>>  void kvm_timer_flush_hwstate(struct kvm_vcpu *vcpu);
>>  void kvm_timer_sync_hwstate(struct kvm_vcpu *vcpu);
>> +bool kvm_timer_should_notify_user(struct kvm_vcpu *vcpu);
>> +void kvm_timer_update_run(struct kvm_vcpu *vcpu);
>>  void kvm_timer_vcpu_terminate(struct kvm_vcpu *vcpu);
>>
>>  u64 kvm_arm_timer_get_reg(struct kvm_vcpu *, u64 regid);
>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
>> index 363f0d2..5dc2167 100644
>> --- a/virt/kvm/arm/arch_timer.c
>> +++ b/virt/kvm/arm/arch_timer.c
>> @@ -184,6 +184,27 @@ bool kvm_timer_should_fire(struct arch_timer_context *timer_ctx)
>>  	return cval <= now;
>>  }
>>
>> +/*
>> + * Reflect the timer output level into the kvm_run structure
>> + */
>> +void kvm_timer_update_run(struct kvm_vcpu *vcpu)
>> +{
>> +	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
>> +	struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);
>> +	struct kvm_sync_regs *regs = &vcpu->run->s.regs;
>> +
>> +	if (likely(irqchip_in_kernel(vcpu->kvm)))
>> +		return;
>> +
>> +	/* Populate the device bitmap with the timer states */
>> +	regs->device_irq_level &= ~(KVM_ARM_DEV_EL1_VTIMER |
>> +				    KVM_ARM_DEV_EL1_PTIMER);
>> +	if (vtimer->irq.level)
>> +		regs->device_irq_level |= KVM_ARM_DEV_EL1_VTIMER;
>> +	if (ptimer->irq.level)
>> +		regs->device_irq_level |= KVM_ARM_DEV_EL1_PTIMER;
>> +}
>> +
>>  static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
>>  				 struct arch_timer_context *timer_ctx)
>>  {
>> @@ -194,9 +215,12 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
>>  	trace_kvm_timer_update_irq(vcpu->vcpu_id, timer_ctx->irq.irq,
>>  				   timer_ctx->irq.level);
>>
>> -	ret = kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id, timer_ctx->irq.irq,
>> -				  timer_ctx->irq.level);
>> -	WARN_ON(ret);
>> +	if (likely(irqchip_in_kernel(vcpu->kvm))) {
>> +		ret = kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
>> +					  timer_ctx->irq.irq,
>> +					  timer_ctx->irq.level);
>> +		WARN_ON(ret);
>> +	}
>>  }
>>
>>  /*
>> @@ -215,7 +239,7 @@ static void kvm_timer_update_state(struct kvm_vcpu *vcpu)
>>  	 * because the guest would never see the interrupt.  Instead wait
>>  	 * until we call this function from kvm_timer_flush_hwstate.
>>  	 */
>> -	if (!timer->enabled)
>> +	if (unlikely(!timer->enabled))
>>  		return;
>>
>>  	if (kvm_timer_should_fire(vtimer) != vtimer->irq.level)
>> @@ -282,28 +306,12 @@ void kvm_timer_unschedule(struct kvm_vcpu *vcpu)
>>  	timer_disarm(timer);
>>  }
>>
>> -/**
>> - * kvm_timer_flush_hwstate - prepare to move the virt timer to the cpu
>> - * @vcpu: The vcpu pointer
>> - *
>> - * Check if the virtual timer has expired while we were running in the host,
>> - * and inject an interrupt if that was the case.
>> - */
>> -void kvm_timer_flush_hwstate(struct kvm_vcpu *vcpu)
>> +static void kvm_timer_flush_hwstate_vgic(struct kvm_vcpu *vcpu)
>>  {
>> -	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
>>  	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
>>  	bool phys_active;
>>  	int ret;
>>
>> -	if (unlikely(!timer->enabled))
>> -		return;
>> -
>> -	kvm_timer_update_state(vcpu);
>> -
>> -	/* Set the background timer for the physical timer emulation. */
>> -	kvm_timer_emulate(vcpu, vcpu_ptimer(vcpu));
>> -
>>  	/*
>>  	* If we enter the guest with the virtual input level to the VGIC
>>  	* asserted, then we have already told the VGIC what we need to, and
>> @@ -355,11 +363,72 @@ void kvm_timer_flush_hwstate(struct kvm_vcpu *vcpu)
>>  	vtimer->active_cleared_last = !phys_active;
>>  }
>>
>> +bool kvm_timer_should_notify_user(struct kvm_vcpu *vcpu)
>> +{
>> +	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
>> +	struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);
>> +	struct kvm_sync_regs *sregs = &vcpu->run->s.regs;
>> +	bool vlevel, plevel;
>> +
>> +	if (likely(irqchip_in_kernel(vcpu->kvm)))
>> +		return false;
>> +
>> +	vlevel = sregs->device_irq_level & KVM_ARM_DEV_EL1_VTIMER;
>> +	plevel = sregs->device_irq_level & KVM_ARM_DEV_EL1_PTIMER;
>> +
>> +	return vtimer->irq.level != vlevel ||
>> +	       ptimer->irq.level != plevel;
>> +}
>> +
>> +static void kvm_timer_flush_hwstate_user(struct kvm_vcpu *vcpu)
>> +{
>> +	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
>> +
>> +	/*
>> +	 * To prevent continuously exiting from the guest, we mask the
>> +	 * physical interrupt such that the guest can make forward progress.
>> +	 * Once we detect the output level being deasserted, 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);
>> +}
>> +
>> +/**
>> + * kvm_timer_flush_hwstate - prepare timers before running the vcpu
>> + * @vcpu: The vcpu pointer
>> + *
>> + * Check if the virtual timer has expired while we were running in the host,
>> + * and inject an interrupt if that was the case, making sure the timer is
>> + * masked or disabled on the host so that we keep executing.  Also schedule a
>> + * software timer for the physical timer if it is enabled.
>> + */
>> +void kvm_timer_flush_hwstate(struct kvm_vcpu *vcpu)
>> +{
>> +	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
>> +
>> +	if (unlikely(!timer->enabled))
>> +		return;
>> +
>> +	kvm_timer_update_state(vcpu);
>> +
>> +	/* Set the background timer for the physical timer emulation. */
>> +	kvm_timer_emulate(vcpu, vcpu_ptimer(vcpu));
>> +
>> +	if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
>> +		kvm_timer_flush_hwstate_user(vcpu);
>> +	else
>> +		kvm_timer_flush_hwstate_vgic(vcpu);
>> +}
>> +
>>  /**
>>   * kvm_timer_sync_hwstate - sync timer state from cpu
>>   * @vcpu: The vcpu pointer
>>   *
>> - * Check if the virtual timer has expired while we were running in the guest,
>> + * Check if any of the timers have expired while we were running in the guest,
>>   * and inject an interrupt if that was the case.
>>   */
>>  void kvm_timer_sync_hwstate(struct kvm_vcpu *vcpu)
>> @@ -559,6 +628,13 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
>>  	if (timer->enabled)
>>  		return 0;
>>
>> +	/* Without a VGIC we do not map virtual IRQs to physical IRQs */
>> +	if (!irqchip_in_kernel(vcpu->kvm))
>> +		goto no_vgic;
>> +
>> +	if (!vgic_initialized(vcpu->kvm))
>> +		return -ENODEV;
>> +
>>  	/*
>>  	 * Find the physical IRQ number corresponding to the host_vtimer_irq
>>  	 */
>> @@ -582,8 +658,8 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
>>  	if (ret)
>>  		return ret;
>>
>> +no_vgic:
>>  	timer->enabled = 1;
> 
> What happens if
> 
>    1) User space spawns a VM with user space irqchip
>    2) Runs the VM
>    3) Then adds a virtual gic device

As soon as a vcpu has run once, it is not possible to instantiate a vgic
(see virt/kvm/arm/vgic/vgic-init.c:kvm_vgic_create around line 101).

Thanks,

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