[vfio-users] Q on vfio-pci driver usage on Host

[Alex Williamson]

On Mon, 13 Apr 2020 10:33:21 -0700
Ravi Kerur <rkerur at gmail.com> wrote:

> On Mon, Apr 13, 2020 at 8:36 AM Alex Williamson <alex.williamson at redhat.com>
> wrote:
> 
> > On Sun, 12 Apr 2020 09:10:49 -0700
> > Ravi Kerur <rkerur at gmail.com> wrote:
> >  
> > > Hi,
> > >
> > > I use Intel NICs for PF and VF devices. VFs are assigned to virtual
> > > machines and PF is used on the Host. I have intel-iommu=on on GRUB which
> > > enables DMAR and IOMMU capabilities (checked via 'dmesg | grep -e IOMMU  
> > -e  
> > > DMAR) and use DPDK for datapath acceleration.
> > >
> > > Couple of clarifications I need in terms of vfio-pci driver usage
> > >
> > > (1) with intel-iommu=pt (Passthrough mode), PF device on host can bind to
> > > either igb_uio or vfio-pci driver and similarly VF devices assigned to  
> > each  
> > > VM can bind to either igb_uio or vfio-pci driver via Qemu  
> >
> > Note that the actual option is 'intel_iommu=on iommu=pt'.
> >  
> 
> My mistake,
> 
> >  
> > > (2) with intel-iommu=on (IOMMU enabled), PF device on host must bind to
> > > vfio-pci driver and similarly VF devices assigned to each VM much bind to
> > > vfio-pci driver. When IOMMU is enabled, only vfio-pci should be used?  
> >
> > When an IOMMU is present, we refer to the address space through which a
> > device performs DMA as the I/O Virtual Address space, or IOVA.  When
> > the IOMMU is in passthrough mode, we effectively create an identity
> > mapping of physical addresses through the IOVA space.  Therefore to
> > program a device to perform a DMA to user memory, the user only needs
> > to perform a virtual to physical translation on the address and the
> > device can DMA directly with that physical address thanks to the
> > identity map.  When we're not in passthrough mode, we need to actually
> > create a mapping through the IOMMU to allow the device to access that
> > physical memory.  VFIO is the only userspace driver interface that I'm
> > aware of that provides this latter functionality.  Therefore, yes, if
> > you have the IOMMU enabled and not in passthrough mode, your userspace
> > driver needs support for programming the IOMMU, which vfio-pci provides.
> >
> > Also, having both the PF and VFs owned by userspace drivers presents
> > some additional risks, for example the PF may have access to the data
> > accessed by the VF, or at least be able to deny service to the VF.
> > There have been various hacks around this presented by the DPDK
> > community, essentially enabling SR-IOV underneath vfio-pci, without the
> > driver's knowledge.  These are very much dis-recommended, IMO.
> > However, we have added SR-IOV support to the vfio-pci driver in kernel
> > v5.7 and DPDK support is under development, which represents this trust
> > and collaboration between PF and VF drivers using a new VF token
> > concept.  I'd encourage you to look for this if your configuration does
> > require both PF and VF drivers in userspace.  A much more normal
> > configuration to this point has been that the PF makes use of a host
> > driver (ex. igb, ixgbe, i40e, etc.) while the VF is bound to vfio-pci
> > for userspace drivers.  In this configuration the host driver is
> > considered trusted and we don't need to invent new mechanisms to
> > indicate collaboration between userspace drivers.  Thanks,
> >  
> 
> Thanks for the information. Clearly understand what I need to do. Where can
> I find information on vfio-pci sr-iov support (writeup/design)?

https://lore.kernel.org/kvm/158396044753.5601.14804870681174789709.stgit@gimli.home/

http://mails.dpdk.org/archives/dev/2020-April/163095.html

Thanks,
Alex