Re: [RFC PATCH 0/7] vfio/pci: SR-IOV support

[Alex Williamson]

On Wed, 5 Feb 2020 07:57:21 +0000
"Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote:

> Hi Alex,
> 
> Silly questions on the background:
> 
> > From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Sent: Wednesday, February 5, 2020 7:06 AM
> > Subject: [RFC PATCH 0/7] vfio/pci: SR-IOV support
> > 
> > There seems to be an ongoing desire to use userspace, vfio-based
> > drivers for both SR-IOV PF and VF devices.   
> 
> Is this series to make PF be bound-able to vfio-pci even SR-IOV is
> enabled on such PFs? If yes, is it allowed to assign PF to a VM? or
> it can only be used by userspace applications like DPDK?

No, this series does not change the behavior of vfio-pci with respect
to probing a PF where VFs are already enabled.  This is still
disallowed.  I haven't seen a use case that requires this and allowing
it tends to subvert the restrictions here.  For instance, if an
existing VF is already in use by a vfio-pci driver, the PF can
transition from a trusted host driver to an unknown userspace driver.

> > The fundamental issue
> > with this concept is that the VF is not fully independent of the PF
> > driver.  Minimally the PF driver might be able to deny service to the
> > VF, VF data paths might be dependent on the state of the PF device,
> > or the PF my have some degree of ability to inspect or manipulate the
> > VF data.  It therefore would seem irresponsible to unleash VFs onto
> > the system, managed by a user owned PF.
> > 
> > We address this in a few ways in this series.  First, we can use a bus
> > notifier and the driver_override facility to make sure VFs are bound
> > to the vfio-pci driver by default.  This should eliminate the chance
> > that a VF is accidentally bound and used by host drivers.  We don't
> > however remove the ability for a host admin to change this override.
> > 
> > The next issue we need to address is how we let userspace drivers
> > opt-in to this participation with the PF driver.  We do not want an
> > admin to be able to unwittingly assign one of these VFs to a tenant
> > that isn't working in collaboration with the PF driver.  We could use
> > IOMMU grouping, but this seems to push too far towards tightly coupled
> > PF and VF drivers.  This series introduces a "VF token", implemented
> > as a UUID, as a shared secret between PF and VF drivers.  The token
> > needs to be set by the PF driver and used as part of the device
> > matching by the VF driver.  Provisions in the code also account for
> > restarting the PF driver with active VF drivers, requiring the PF to
> > use the current token to re-gain access to the PF.  
> 
> How about the scenario in which PF driver is vfio-based userspace
> driver but VF drivers are mixed. This means not all VFs are bound
> to vfio-based userspace driver. Is it also supported here? :-)

It's allowed.  Userspace VF drivers will need to participate in the VF
token scheme, host drivers may be bound to VFs normally after removing
the default driver_override.  Thanks,

Alex