On Thu, 2011-06-30 at 15:59 +0000, Yoder Stuart-B08248 wrote:
One feature we need for QEMU/KVM on embedded Power Architecture is the
ability to do passthru assignment of SoC I/O devices and memory. An
important use case in embedded is creating static partitions--
taking physical memory and I/O devices (non-PCI) and partitioning
them between the host Linux and several virtual machines. Things like
live migration would not be needed or supported in these types of scenarios.
SoC devices do not sit on a probeable bus and there are no identifiers
like 01:00.0 with PCI that we can use to identify devices-- the host
Linux kernel is made aware of SoC I/O devices from nodes/properties in a
device tree structure passed at boot. QEMU needs to generate a
device tree to pass to the guest as well with all the guest's virtual
and physical resources. Today a number of mostly complete guest device
trees are kept under ./pc-bios in QEMU, but this too static and
inflexible.
Some new mechanism is needed to assign SoC devices to guests, and we
(FSL + Alex Graf) have been discussing a few possible approaches
for doing this from QEMU and would like some feedback.
Some possibilities:
1. Option 1. Pass the host dev tree to QEMU and assign devices
by device tree path
-dtb ./mpc8572ds.dtb -device assigned-soc-dev,dev=/soc/address@hidden
/soc/address@hidden is the device tree path to the assigned device.
The device node 'address@hidden' has some number of properties (e.g.
address, interrupt info) and possibly subnodes under
it. QEMU copies that node when generating the guest dev tree.
See snippet of entire node: http://paste2.org/p/1496460
Yuck (see below)
2. Option 2. Pass the entire assigned device node as a string to
QEMU
-device assigned-soc-dev,dev=/address@hidden,dev-node='#address-cells =<1>;
#size-cells =<0>; cell-index =<0>; compatible = "fsl-i2c";
reg =<0xffe03000 0x100>; interrupts =<43 2>;
interrupt-parent =<&mpic>; dfsrr;'
Beuark ! (see below)
This avoids needing to pass the host device tree, but could
get awkward-- the i2c example above is very simple, some device
nodes are very large with a complex hierarchy of subnodes and
could be hundreds of lines of text to represent a single
node.
It gets more complicated...
So, from a qemu command line perspective, all you should have to do is
pass qemu the device-tree -path- to the device you want to pass-trough
(you may support passing a full hierarchy here).
That is for normal MMIO mapped SoC devices. Something else (individual
i2c, usb, ...) will use specific virtualization of the corresponding
busses.
Anything else sucks too much really.
From there, well, there's several approach inside qemu/kvm to handle
that path. If you want to do things at the qemu level you can probably
parse /proc/device-tree. But I'd personally just make it a kernel thing.
IE. I would have an ioctl to "instanciate" a pass-through device, that
takes that path as an argument. I would make it return an anonymous fd
which you can then use to mmap the resources, etc...