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

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Message-ID: <20160426140816.67b8b37c@t450s.home>
Date:	Tue, 26 Apr 2016 14:08:16 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Feng Wu <feng.wu@...el.com>
Cc:	pbonzini@...hat.com, joro@...tes.org, mtosatti@...hat.com,
	eric.auger@...aro.org, 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

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,

Alex