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Date:   Thu, 5 Mar 2020 11:22:30 -0700
From:   Alex Williamson <alex.williamson@redhat.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
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        "Richardson, Bruce" <bruce.richardson@intel.com>,
        "cohuck@redhat.com" <cohuck@redhat.com>
Subject: Re: [PATCH v2 5/7] vfio/pci: Add sriov_configure support
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On Tue, 25 Feb 2020 03:08:00 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Alex Williamson
> > Sent: Thursday, February 20, 2020 2:54 AM
> > 
> > With the VF Token interface we can now expect that a vfio userspace
> > driver must be in collaboration with the PF driver, an unwitting
> > userspace driver will not be able to get past the GET_DEVICE_FD step
> > in accessing the device.  We can now move on to actually allowing
> > SR-IOV to be enabled by vfio-pci on the PF.  Support for this is not
> > enabled by default in this commit, but it does provide a module option
> > for this to be enabled (enable_sriov=1).  Enabling VFs is rather
> > straightforward, except we don't want to risk that a VF might get
> > autoprobed and bound to other drivers, so a bus notifier is used to
> > "capture" VFs to vfio-pci using the driver_override support.  We
> > assume any later action to bind the device to other drivers is
> > condoned by the system admin and allow it with a log warning.
> > 
> > vfio-pci will disable SR-IOV on a PF before releasing the device,
> > allowing a VF driver to be assured other drivers cannot take over the
> > PF and that any other userspace driver must know the shared VF token.
> > This support also does not provide a mechanism for the PF userspace
> > driver itself to manipulate SR-IOV through the vfio API.  With this
> > patch SR-IOV can only be enabled via the host sysfs interface and the
> > PF driver user cannot create or remove VFs.  
> 
> I'm not sure how many devices can be properly configured simply 
> with pci_enable_sriov. It is not unusual to require PF driver prepare
> something before turning PCI SR-IOV capability. If you look kernel
> PF drivers, there are only two using generic pci_sriov_configure_
> simple (simple wrapper like pci_enable_sriov), while most others
> implementing their own callback. However vfio itself has no idea
> thus I'm not sure how an user knows whether using this option can
> actually meet his purpose. I may miss something here, possibly 
> using DPDK as an example will make it clearer.

There is still the entire vfio userspace driver interface.  Imagine for
example that QEMU emulates the SR-IOV capability and makes a call out
to libvirt (or maybe runs with privs for the PF SR-IOV sysfs attribs)
when the guest enables SR-IOV.  Can't we assume that any PF specific
support can still be performed in the userspace/guest driver, leaving
us with a very simple and generic sriov_configure callback in vfio-pci?
Thanks,

Alex