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    SubjectRe: [PATCH v9 12/18] vfio: Register/unregister irq_bypass_producer
    On Wed, 27 Apr 2016 01:32:32 +0000
    "Wu, Feng" <feng.wu@intel.com> wrote:

    > > -----Original Message-----
    > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
    > > Sent: Wednesday, April 27, 2016 4:08 AM
    > > To: Wu, Feng <feng.wu@intel.com>
    > > Cc: pbonzini@redhat.com; joro@8bytes.org; mtosatti@redhat.com;
    > > eric.auger@linaro.org; kvm@vger.kernel.org; iommu@lists.linux-
    > > foundation.org; linux-kernel@vger.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@intel.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,
    >
    > Can we just return 0 when kvm_x86_ops->update_pi_irte is NULL in
    > kvm_arch_irq_bypass_add_producer?

    Yeah, that may be the best way to go, only return error for actual
    failures, not for simple lack of a bypass mechanism. This is
    consistent with what update_pi_irte does when running on hardware
    or configurations without PI. Thanks,

    Alex

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