linux-kernel - Re: [PATCH] virtio_pci: support enabling VFs

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Message-ID: <1527699273.29907.2.camel@intel.com>
Date:   Wed, 30 May 2018 16:54:35 +0000
From:   "Duyck, Alexander H" <alexander.h.duyck@...el.com>
To:     "Rustad, Mark D" <mark.d.rustad@...el.com>,
        "mst@...hat.com" <mst@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "Daly, Dan" <dan.daly@...el.com>,
        "Bie, Tiwei" <tiwei.bie@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "Liang, Cunming" <cunming.liang@...el.com>,
        "Wang, Zhihong" <zhihong.wang@...el.com>
Subject: Re: [PATCH] virtio_pci: support enabling VFs

On Wed, 2018-05-30 at 09:44 -0700, Rustad, Mark D wrote:
> On May 30, 2018, at 9:22 AM, Michael S. Tsirkin <mst@...hat.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.

Why even have the call though? I would wrap all of this in an #ifdef
and strip it out since you couldn't support SR-IOV if it isn't present
in the kernel anyway.