[Qemu-devel] vfio for platform devices - 9/5/2012 - minutes

[Yoder Stuart-B08248]

We had a call with those interested and/or working on vfio
for platform devices.

Participants: Scott Wood, Varun Sethi, Bharat Bhushan, Peter Maydell,
              Santosh Shukla, Alex Williamson, Alexander Graf,
              Antonios Motakis, Christoffer Dall, Kim Phillips,
              Stuart Yoder

Several aspects to vfio for platform devices:

1. IOMMU groups

 -iommu driver needs to register a bus notifier for the platform bus
  and create groups for relevant platform devices
 -Antonios is looking at this for several ARM IOMMUs
 -PAMU (Freescale) driver already does this

2. unbinding device from host

 PCI:
   echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
 Platform:
   echo ffe101300.dma > /sys/bus/platform/devices/ffe101300.dma/driver/unbind

 -don't believe there are issues or work to do here

3. binding device to vfio-platform driver

 PCI:
   echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id

 -this is probably the least understood issue-- platform drivers
  register themselves with the bus for a specific "name"
  string.  That is matched with device tree "compatible" strings
  later to bind a device to a driver
 -we want is to have the vfio-platform driver to dynamically bind
  to a variety of platform devices previously unknown to
  vfio-platform
 -ideally unbinding and binding could be an atomic operation
 -Alex W pointed out that x86 could leverage this work so
  keep that in mind in what we design
 -Kim Phillips (Linaro) will start working on this

4. vfio kernel interface

 -exposes regions and interrupts to user space via FDs
 -there are 'info' ioctls that allow getting info about
  regions/interrupts such as size and type of interrupt
 -there is a proposed extension to the 'info' ioctls that
  provides device tree paths, allowing user space to coorelate
  resources with the device tree
  (https://lists.cs.columbia.edu/pipermail/kvmarm/2013-July/006237.html)

5. QEMU
 -some key tasks
   -interacts with vfio kernel interface
   -registers memslots
   -needs to dynamically get device hooked up to VM's
    platform bus, including IRQs
   -needs to generate device tree node
 -a key point: we don't believe that platform device passthru
  in QEMU can be solved in a completely generic way.  There will
  need to be device specific code for each device type being passed
  through...to do things like generate device tree nodes
 -in general we expect a relatively small number of device types
  to be passed through to VMs
 -Alex Graf is working on dynamic creation of platform devices
  for the PPC e500 paravirt machine
 -see: http://lists.nongnu.org/archive/html/qemu-devel/2013-07/msg03614.html
 -first step is dynamically generating a virtual UART
 -that sets the stage to create and create vfio devices backed
  by real hardware

There is a session at Linux Plumbers in a couple of weeks
and further discussions will happen there.

Regards,
Stuart