I don't know anything about your specific hardware, but it is true
that secondary GPUs are often not connected to the display itself, but
rather the rendering takes place there and then the rendered frames
are passed back to the host and to the integrated gpu to be put on
your display. From a Qubes perspective I believe this is actually a
very good thing since it means we could keep the integrated GPU
statically assigned to dom0, and keep the qubes gui protocol largely
unchanged. The question would be one of getting the passed through GPU
to render its output to some buffer which we pass back to dom0.

There are still firmware-security issues associated with passing the
discrete GPU between VMs of different trust levels, because someone
who has full control of the GPU may be able to re-flash its firmware
with something that would later perform a DMA attack against the 2nd
VM it's attached to. However, if you only ever wish to pass it through
to a single "gaming" windows HVM or such, this is not a problem.

The reason integrated GPUs are interesting in this regard is that they
do not have firmware which is persistently stored on the device,
rather it is loaded externally on each power-on and subject to normal
boot-security measures. The thinking is that by rebooting between
assigning your integrated GPU to different VMs, you prevent one from
compromising another via the GPU by making GPU compromise ephemeral.

As for previous successes requiring upstream-QEMU in dom0, the problem
here is that Xen only supports a very old forked QEMU in stubdomains,
but this is something that will change. Progress in this area has
stalled because there was an effort to run QEMU in a very minimal
unikernel-style environment, but this effort has been abandoned and
work is now underway towards making it run on top of linux (still in a
separate stubdomain), which should take less work to bring to a usable
state than the previous minimal-stubdom effort.

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