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

[Jike Song <jike.song@intel.com>]

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
>> hypervisor
>>      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:
>>
>>                  kvm_io_bus_register_dev
>>                  install_new_memslots
>>
>>          must be called *without* &kvm->srcu read-locked. Otherwise it
>> hangs.
>>
>>          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
>> functions.
>>
>>          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
>> like
>>          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:
>>
>>      git://github.com/01org/KVMGT-kernel
>>      git://github.com/01org/KVMGT-qemu
>>      git://github.com/01org/KVMGT-seabios
>>
>> 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:
>>
>>      https://www.usenix.org/conference/atc14/technical-sessions/presentation/tian
>>
>>      http://events.linuxfoundation.org/sites/events/files/slides/KVMGT-a%20Full%20GPU%20Virtualization%20Solution_1.pdf
>>
>>
>>
>> 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?
>
> Jan
>

--
Thanks,
Jike
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