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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Alex Williamson <alex.williamson@redhat.com>
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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
Thread-Topic: [PATCH v2 5/7] vfio/pci: Add sriov_configure support
Thread-Index: AQHV51YaoTnickP570etlLGInd5eAKgrQJIwgA6gtQCAAWITYA==
Date:   Fri, 6 Mar 2020 07:57:19 +0000
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> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Friday, March 6, 2020 2:23 AM
> 
> 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?

Makes sense. One concern, though, is how an user could be warned
if he inadvertently uses sysfs to enable SR-IOV on a vfio device whose
userspace driver is incapable of handling it. Note any VFIO device, 
if SR-IOV capable, will allow user to do so once the module option is 
turned on and the callback is registered. I felt such uncertainty can be 
contained by toggling SR-IOV through a vfio api, but from your description 
obviously it is what you want to avoid. Is it due to the sequence reason,
e.g. that SR-IOV must be enabled before userspace PF driver sets the 
token? 

Thanks
Kevin