netdev - Re: [PATCH] pci-iov: Add support for unmanaged SR-IOV

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Date:   Fri, 2 Mar 2018 11:14:11 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        "Daly, Dan" <dan.daly@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        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>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>,
        Ilya Lesokhin <ilyal@...lanox.com>,
        Don Dutile <ddutile@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>
Subject: Re: [PATCH] pci-iov: Add support for unmanaged SR-IOV

On Fri, 2 Mar 2018 06:54:17 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > From: Alex Williamson
> > Sent: Friday, March 2, 2018 4:22 AM  
> > >
> > > I am pretty sure that you are describing is true of some, but not for
> > > all. I think the Amazon solutions and the virtio solution are doing
> > > hard partitioning of the part. I will leave it to those guys to speak
> > > for themselves since I don't know anything about the hardware design
> > > of those parts.  
> > 
> > I think we'd need device specific knowledge and enablement to be able
> > to take advantage of any hardware partitioning, otherwise we need to
> > assume the pf is privileged, as implemented in other sriov devices.
> > 
> > I'm also trying to imagine whether there's a solution via the new
> > vfio/mdev interface, where the mdev vendor driver would bind to the pf
> > and effectively present itself as the mdev device.  The vendor driver
> > could provide sriov capabilities and bear the burden of ensuring that
> > the pf is used cooperatively.  The only existing mdev vendor drivers are
> > vGPUs and rely on on-device DMA translation and isolation, such as
> > through GTTs, but there have been some thoughts on providing IOMMU
> > based
> > isolation of mdev/sriov mixed devices (assuming DMA is even required
> > for userspace management of the pf in this use case).  [Cc Kirti]
> > Thanks,
> >   
> 
> Hope not distracting this thread, but above sounds like an interesting
> idea. Actually we ever brainstormed similar thought for another 
> potential usage - supporting VF live migration. We are already working
> on an generic extension to allow state save/restore of mdev instance.
> If vendor driver could further wrap pf as a mdev instance, it could 
> leverage the same framework for a clean state migration on VF. based
> on mmap callback the vendor driver can easily switch back-and-forth
> between pass through and trap/emulation of the VF resources. Of
> course doing so alone doesn't address all the demands of VF live
> migration (e.g. dirty page tracking still requires other techniques), 
> but it does pave a way toward a general framework to support VF
> live migration.
> 
> If above is feasible, finally we could use one mdev framework to
> manage both mdev and pf/vf assignment, while providing added
> values which are difficult to achieve today. :-)

mdev drivers may be the first to support migration, but I wonder if a
full mdev implementation is necessary for it.  Once the vfio api is
define, device specific extensions to vfio-pci might be able to
implement migration more directly.  Thanks,

Alex