Re: [Qemu-devel] [Intel-gfx] [Announcement] 2015-Q3 release of XenGT - a Mediated Graphics Passthrough Solution from Intel

From: Jike Song
Date: Sun Nov 22 2015 - 23:52:53 EST

On 11/21/2015 12:40 AM, Alex Williamson wrote:

Thanks for confirmation. For QEMU/KVM, I totally agree your point; However,
if we take XenGT to consider, it will be a bit more complex: with Xen
hypervisor and Dom0 kernel running in different level, it's not a straight-
forward way for QEMU to do something like mapping a portion of MMIO BAR
via VFIO in Dom0 kernel, instead of calling hypercalls directly.

This would need to be part of the support added for Xen. To directly
map a device MMIO space to the VM, VFIO provides an mmap, QEMU registers
that mmap with KVM, or Xen. It's all just MemoryRegions in QEMU.
Perhaps it's even already supported by Xen.

AFAICT, things are different here for Xen. To establish mappings between
Dom0 pfns and DomU gfn, one will have to call Xen hypercalls. In the scene
above, either QEMU calls the hypercall directly, or it asks the VFIO in
dom0 kernel to do it.

I'm not saying that VFIO is not applicable for XenGT. I just want to
say that given the VFIO based kernel/QEMU split model, additional effort
is needed for XenGT.

I don't know if there is a better way to handle this. But I do agree that
channels between kernel and Qemu via VFIO is a good idea, even though we
may have to split KVMGT/XenGT in Qemu a bit. We are currently working on
moving all of PCI CFG emulation from kernel to Qemu, hopefully we can
release it by end of this year and work with you guys to adjust it for
the agreed method.

Well, moving PCI config space emulation from kernel to QEMU is exactly
the wrong direction to take for this proposal. Config space access to
the vGPU would occur through the VFIO API. So if you already have
config space emulation in the kernel, that's already one less piece of
work for a VFIO model, it just needs to be "wired up" through the VFIO
API. Thanks,

If I understand correctly, the idea of moving PCI CFG to QEMU is actually
very similar to your VFIO design:

a) VM access a CFG regsiter
b) KVM hands over the access to QEMU
c) QEMU may emulate it, and when necessary, ioctl into kernel(i915/vgt)


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