linux-kernel - Re: [virtio-dev] [pci PATCH v7 2/5] virtio_pci: Add support for unmanaged SR-IOV on virtio

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Message-ID: <20180403161151-mutt-send-email-mst@kernel.org>
Date:   Tue, 3 Apr 2018 16:12:39 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     "Daly, Dan" <dan.daly@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        linux-pci@...r.kernel.org, virtio-dev@...ts.oasis-open.org,
        kvm@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org,
        Keith Busch <keith.busch@...el.com>, netanel@...zon.com,
        Don Dutile <ddutile@...hat.com>,
        Maximilian Heyne <mheyne@...zon.de>,
        "Wang, Liang-min" <liang-min.wang@...el.com>,
        "Rustad, Mark D" <mark.d.rustad@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Christoph Hellwig <hch@....de>, dwmw@...zon.co.uk
Subject: Re: [virtio-dev] [pci PATCH v7 2/5] virtio_pci: Add support for
 unmanaged SR-IOV on virtio_pci devices

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@...hat.com> wrote:
> > On Thu, Mar 15, 2018 at 11:42:41AM -0700, Alexander Duyck wrote:
> >> From: Alexander Duyck <alexander.h.duyck@...el.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@....de>
> >> Signed-off-by: Mark Rustad <mark.d.rustad@...el.com>
> >> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.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.


> One of the reasons why Mark called out the
> subvendor/subdevice was because that might be able to be used to
> identify the specific hardware that is providing the SR-IOV feature so
> in the future if it is added to virtio itself then you could exclude
> devices like this by just limiting things based on subvendor/subdevice
> IDs.
> 
> > Also, I am guessing that hardware implementations will want
> > to add things like stong memory barriers - I guess we
> > will add new feature bits for that too down the road?
> 
> That piece I don't have visibility into at this time. Perhaps Dan
> might have more visibility into future plans on what this might need.
> 
> Thanks.
> 
> - Alex