Re: [Intel-gfx] [ANNOUNCE][RFC] KVMGT - the implementation of Intel GVT-g(full GPU virtualization) for KVM

From: Jike Song
Date: Wed Dec 10 2014 - 01:37:41 EST

CC Kevin.

On 12/09/2014 05:54 PM, Jan Kiszka wrote:
On 2014-12-04 03:24, Jike Song wrote:
Hi all,

We are pleased to announce the first release of KVMGT project. KVMGT is
the implementation of Intel GVT-g technology, a full GPU virtualization
solution. Under Intel GVT-g, a virtual GPU instance is maintained for
each VM, with part of performance critical resources directly assigned.
The capability of running native graphics driver inside a VM, without
hypervisor intervention in performance critical paths, achieves a good
balance of performance, feature, and sharing capability.

KVMGT is still in the early stage:

- Basic functions of full GPU virtualization works, guest can see a
full-featured vGPU.
We ran several 3D workloads such as lightsmark, nexuiz, urbanterror
and warsow.

- Only Linux guest supported so far, and PPGTT must be disabled in
guest through a
kernel parameter(see README.kvmgt in QEMU).

- This drop also includes some Xen specific changes, which will be
cleaned up later.

- Our end goal is to upstream both XenGT and KVMGT, which shares ~90%
logic for vGPU
device model (will be part of i915 driver), with only difference in
specific services

- insufficient test coverage, so please bear with stability issues :)

There are things need to be improved, esp. the KVM interfacing part:

1 a domid was added to each KVMGT guest

An ID is needed for foreground OS switching, e.g.

# echo <domid> > /sys/kernel/vgt/control/foreground_vm

domid 0 is reserved for host OS.

2 SRCU workarounds.

Some KVM functions, such as:


must be called *without* &kvm->srcu read-locked. Otherwise it

In KVMGT, we need to register an iodev only *after* BAR
registers are
written by guest. That means, we already have &kvm->srcu hold -
trapping/emulating PIO(BAR registers) makes us in such a condition.
That will make kvm_io_bus_register_dev hangs.

Currently we have to disable rcu_assign_pointer() in such

These were dirty workarounds, your suggestions are high welcome!

3 syscalls were called to access "/dev/mem" from kernel

An in-kernel memslot was added for aperture, but using syscalls
open and mmap to open and access the character device "/dev/mem",
for pass-through.

The source codes(kernel, qemu as well as seabios) are available at github:


In the KVMGT-qemu repository, there is a "README.kvmgt" to be referred.

More information about Intel GVT-g and KVMGT can be found at:

Appreciate your comments, BUG reports, and contributions!

There is an even increasing interest to keep KVM's in-kernel guest
interface as small as possible, specifically for security reasons. I'm
sure there are some good performance reasons to create a new in-kernel
device model, but I suppose those will need good evidences why things
are done in the way they finally should be - and not via a user-space
device model. This is likely not a binary decision (all userspace vs. no
userspace), it is more about the size and robustness of the in-kernel
model vs. its performance.

One aspect could also be important: Are there hardware improvements in
sight that will eventually help to reduce the in-kernel device model and
make the overall design even more robust? How will those changes fit
best into a proposed user/kernel split?


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