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

[Marc Zyngier]

On 31/01/18 10:05, Christoffer Dall wrote:
> On Wed, Jan 31, 2018 at 09:37:31AM +0000, Marc Zyngier wrote:
>> 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.
> 
> this should also be "and"
> 
>>>
>>> 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))
>> 		[...]
>>
> 
> yes, good idea, I find it more readable too.
> 
>>>  
>>>  	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.
>>
> 
> yes
> 
>>>  
>>>  	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
> 
> and here I should s/we must//
> 
>>> +	 * 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()?
>>
> 
> yes, will be fixes if using a static key.
> 
>>> +		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()?
>>
> 
> same
> 
>>> +			enable_percpu_irq(host_vtimer_irq, host_vtimer_irq_flags);
> 
> So, reading your reply made me think.  Shouldn't this actually be:
> 
> 		if (static_key_likely(has_gic_active_state))
> 			enable_percpu_irq(host_vtimer_irq, host_vtimer_irq_flags);
> 		else
> 			irq_set_irqchip_state(host_vtimer_irq, IRQCHIP_STATE_ACTIVE, false);

Shouldn't it be the other way around? Use the active state when we have
a GIC and mask if we don't?

> 
> ... which makes me want to wrap the irqchip call in a small wrapper
> called from here and from load().  What do you think?

Indeed. Some operation that "does the right thing", no matter what what
bizarre configuration we're in. It would definitely help understanding
the flow which has become a bit unwieldy.

Thanks,

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