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From:   "Rustad, Mark D" <mark.d.rustad@intel.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
CC:     "Bie, Tiwei" <tiwei.bie@intel.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "virtio-dev@lists.oasis-open.org" <virtio-dev@lists.oasis-open.org>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "Daly, Dan" <dan.daly@intel.com>,
        "Duyck, Alexander H" <alexander.h.duyck@intel.com>,
        "Liang, Cunming" <cunming.liang@intel.com>,
        "Wang, Zhihong" <zhihong.wang@intel.com>
Subject: Re: [PATCH] virtio_pci: support enabling VFs
Thread-Topic: [PATCH] virtio_pci: support enabling VFs
Thread-Index: AQHT+DJkllI7MbkD40qnnyH4gxudsqRI78oA
Date:   Wed, 30 May 2018 16:44:07 +0000
Message-ID: <414C18B1-30FA-4AC0-B47D-F0FBF9832737@intel.com>
References: <20180530085521.26583-1-tiwei.bie@intel.com>
 <20180530192010-mutt-send-email-mst@kernel.org>
In-Reply-To: <20180530192010-mutt-send-email-mst@kernel.org>
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On May 30, 2018, at 9:22 AM, Michael S. Tsirkin <mst@redhat.com> wrote:

>> +static int virtio_pci_sriov_configure(struct pci_dev *pci_dev, int  
>> num_vfs)
>> +{
>> +	struct virtio_pci_device *vp_dev = pci_get_drvdata(pci_dev);
>> +	struct virtio_device *vdev = &vp_dev->vdev;
>> +	int (*sriov_configure)(struct pci_dev *pci_dev, int num_vfs);
>> +
>> +	if (!(vdev->config->get_status(vdev) & VIRTIO_CONFIG_S_DRIVER_OK))
>> +		return -EBUSY;
>> +
>> +	if (!__virtio_test_bit(vdev, VIRTIO_F_SR_IOV))
>> +		return -EINVAL;
>> +
>> +	sriov_configure = pci_sriov_configure_simple;
>> +	if (sriov_configure == NULL)
>> +		return -ENOENT;
>
> BTW what is all this trickery in aid of?

When SR-IOV support is not compiled into the kernel,  
pci_sriov_configure_simple is #defined as NULL. This allows it to compile  
in that case, even though there is utterly no way for it to be called in  
that case. It is an alternative to #ifs in the code.

--
Mark Rustad, Networking Division, Intel Corporation

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