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Date:   Tue, 3 Apr 2018 21:27:43 +0300
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Alexander Duyck <alexander.duyck@gmail.com>
Cc:     "Daly, Dan" <dan.daly@intel.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        "Duyck, Alexander H" <alexander.h.duyck@intel.com>,
        linux-pci@vger.kernel.org, virtio-dev@lists.oasis-open.org,
        kvm@vger.kernel.org, Netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-nvme@lists.infradead.org,
        Keith Busch <keith.busch@intel.com>, netanel@amazon.com,
        Don Dutile <ddutile@redhat.com>,
        Maximilian Heyne <mheyne@amazon.de>,
        "Wang, Liang-min" <liang-min.wang@intel.com>,
        "Rustad, Mark D" <mark.d.rustad@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Christoph Hellwig <hch@lst.de>, dwmw@amazon.co.uk
Subject: Re: [virtio-dev] [pci PATCH v7 2/5] virtio_pci: Add support for
 unmanaged SR-IOV on virtio_pci devices
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On Tue, Apr 03, 2018 at 10:32:00AM -0700, Alexander Duyck wrote:
> On Tue, Apr 3, 2018 at 6:12 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Fri, Mar 16, 2018 at 09:40:34AM -0700, Alexander Duyck wrote:
> >> On Fri, Mar 16, 2018 at 9:34 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >> > On Thu, Mar 15, 2018 at 11:42:41AM -0700, Alexander Duyck wrote:
> >> >> From: Alexander Duyck <alexander.h.duyck@intel.com>
> >> >>
> >> >> Hardware-realized virtio_pci devices can implement SR-IOV, so this
> >> >> patch enables its use. The device in question is an upcoming Intel
> >> >> NIC that implements both a virtio_net PF and virtio_net VFs. These
> >> >> are hardware realizations of what has been up to now been a software
> >> >> interface.
> >> >>
> >> >> The device in question has the following 4-part PCI IDs:
> >> >>
> >> >> PF: vendor: 1af4 device: 1041 subvendor: 8086 subdevice: 15fe
> >> >> VF: vendor: 1af4 device: 1041 subvendor: 8086 subdevice: 05fe
> >> >>
> >> >> The patch currently needs no check for device ID, because the callback
> >> >> will never be made for devices that do not assert the capability or
> >> >> when run on a platform incapable of SR-IOV.
> >> >>
> >> >> One reason for this patch is because the hardware requires the
> >> >> vendor ID of a VF to be the same as the vendor ID of the PF that
> >> >> created it. So it seemed logical to simply have a fully-functioning
> >> >> virtio_net PF create the VFs. This patch makes that possible.
> >> >>
> >> >> Reviewed-by: Christoph Hellwig <hch@lst.de>
> >> >> Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
> >> >> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> >> >
> >> > So if and when virtio PFs can manage the VFs, then we can
> >> > add a feature bit for that?
> >> > Seems reasonable.
> >>
> >> Yes. If nothing else you may not even need a feature bit depending on
> >> how things go.
> >
> > OTOH if the interface is changed in an incompatible way,
> > and old Linux will attempt to drive the new device
> > since there is no check.
> >
> > I think we should add a feature bit right away.
> 
> I'm not sure why you would need a feature bit. The capability is
> controlled via PCI configuration space. If it is present the device
> has the capability. If it is not then it does not.
> 
> Basically if the PCI configuration space is not present then the sysfs
> entries will not be spawned and nothing will attempt to use this
> function.
> 
> - ALex

It's about compability with older guests which ignore the
capability.

The feature is thus helpful so host knows whether guest supports VFs.


-- 
MSR