linux-kernel - Re: [PATCH v9 12/18] vfio: Register/unregister irq_bypass

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Date:	Thu, 28 Apr 2016 17:35:12 +0200
From:	Eric Auger <eric.auger@...aro.org>
To:	Alex Williamson <alex.williamson@...hat.com>,
	Feng Wu <feng.wu@...el.com>
Cc:	pbonzini@...hat.com, joro@...tes.org, mtosatti@...hat.com,
	kvm@...r.kernel.org, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 12/18] vfio: Register/unregister irq_bypass_producer

Hi Alex,
On 04/26/2016 10:08 PM, Alex Williamson wrote:
> On Fri, 18 Sep 2015 22:29:50 +0800
> Feng Wu <feng.wu@...el.com> wrote:
> 
>  @@ -360,6 +361,14 @@ static int vfio_msi_set_vector_signal(struct
>  vfio_pci_device *vdev,
>>  		return ret;
>>  	}
>>  
>> +	vdev->ctx[vector].producer.token = trigger;
>> +	vdev->ctx[vector].producer.irq = irq;
>> +	ret = irq_bypass_register_producer(&vdev->ctx[vector].producer);
>> +	if (unlikely(ret))
>> +		dev_info(&pdev->dev,
>> +		"irq bypass producer (token %p) registeration fails: %d\n",
>> +		vdev->ctx[vector].producer.token, ret);
>> +
>>  	vdev->ctx[vector].trigger = trigger;
>>  
>>  	return 0;
> 
> Digging back into the IRQ producer/consumer thing, I'm not sure how we
> should be handling a failure here, but it turns out that what we have
> is pretty sub-optimal.  Any sort of testing on AMD hits this dev_info
> because kvm_arch_irq_bypass_add_producer() returns -EINVAL without
> kvm_x86_ops->update_pi_irte which is only implemented for vmx.  Clearly
> we don't want to spew confusing error messages for a feature that does
> not exist.
> 
> The easiest option is to simply make this error silent, but should
> registering a producer/consumer really fail due to a mismatch on the
> other end or should the __connect sequence fail silently, which both
> ends would know about (if they care) due to the add/del handshake
> between them?  Perhaps for now we simply need a stable suitable fix to
> silence the dev_info above, but longer term, registration shouldn't
> fail for mismatches like this.  Thoughts?  Thanks,

Regarding the ARM IRQ forwarding use case, I think it is OK to fail
silently. We would fall back to the irqfd standard mechanism. Anyway
this series still is waiting for ARM new-vgic dependency to be resolved,
as discussed with Christoffer and Marc.

Best Regards

Eric
> 
> Alex
>