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Re: [vfio-users] about hotplug of the IGD device



On Wed, 15 Mar 2017 04:50:40 +0100
Manuel Ullmann <ullman alias posteo de> wrote:

> > Now what I want to do is that rebinding the GPU to the i915 drivers
> > after having started the vm. 
> > Do you have any ways or advices? Thanks.  
> In theory, this would be done with GVT-g. But it is considered unstable
> by its developers and has not a very convincing kernel option help
> [1]. It might also still require patches for Seabios and qemu, although
> I have found a support note for qemu-2.5 regarding Xengt. Most of its
> development happened outside of upstream trees, most likely to get not
> limited by QA measurements.
> 
> Since Alex develops vfio, which tries to prevent device intervention and
> thus makes sharing impossible, but passthrough possible in a quite
> stable way, I’m not sure, whether we would see another helpful blog post
> about this, once it reaches stability.

Heh, I hadn't read the Kconfig option before, it's definitely more than
a stub, but whether it's ready for production is a different story.  As
of v4.10, KVMGT is present and functional, but perhaps not very pretty
or stable.  The benefit and curse of the new vfio mediated device
interface is that it allows exposing software defined devices.  These
can be purely virtual devices in the host kernel, such as the sample
PCI serial device, or portions of real devices, such as we have with
KVMGT and NVIDIA vGPU.

IOMMU-based isolation works when the IOMMU hardware can distinguish
transactions from different devices, which reference a different set of
IOMMU page tables per device.  In the case of a portion of a device, all
transactions look the same, so we can't use the IOMMU for more than
course grained isolation.  This is theoretically not really a problem
when that device is a GPU, because the GPU itself has support for per
process GTTs (Graphics Translation Tables), which provide translation
and isolation between processes, and we can overload this to provide
isolation between each user of said portion of the GPU.

What this boils down to though, and the reason that mediated devices
are a benefit and a curse, is that we have no central, shared, hardware
isolation for the devices.  The "vendor" drivers, as we're calling
them, are responsible for providing the isolation via device specific
means, such as these per-process GTTs.  They also need to make sure
that whatever command channel they use between the virtual device and
the physical device is properly "mediated" such that the virtual device
cannot break out of its isolation or cause other badness (thus the
name of this infrastructure).  The degree of and quality of the
isolation for a mediated device therefore lies squarely on the
vendor driver.  Let's just say that KVMGT has not instilled a great deal
of confidence that such considerations have been fully embodied.  It's
getting better though, the code in v4.11 has improved and we'll see
over time if it really has the robustness that we expect.

It's really quite a cool feature though and as Manuel suggests, it does
obviate the need for re-binding IGD to any drivers.  Simply create the
vGPU type you want, make use of it in the guest while simultaneously
using the physical device in the host, shutdown the VM and remove the
vGPU from the host.  There's not yet any support for connecting a vGPU
to a physical display, for now you'll need to use guest-based remoting
tools (ex. VNC).  Above the kernel, QEMU-2.7 or newer should work just
fine.  libvirt support is under development and will likely be in their
next release, at least for managing VMs with pre-created mediate
devices.  SPICE support is also under development.  I'm not sure what,
if any, plans Intel has for physical displays.  Guest Windows drivers
are not yet released AFAIK and Linux guests should probably use a v4.8
kernel, or perhaps v4.10+ as there was a regression in v4.9 (or please
go find bugs using other kernels).  If you're an early adopter and have
a Broadwell+ system, give it a try, particularly if you're willing to
report bugs on their mailing list <iGVT-g lists 01 org>.  Thanks,

Alex


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