Re: [Qemu-devel] Better Cortex-M support?

[Peter Maydell <peter.maydell@linaro.org>]

On 29 October 2014 13:28, Christopher Covington <cov@codeaurora.org> wrote:
> I've sometimes thought it might be cool if QEMU could consume a DTB and
> emulate whatever is described, assuming the devices and configurations are
> supported. I've yet to come up with a real problem to motivate this
> "solution", though.

This has been suggested before, but unfortunately it's not really
feasible except in certain constrained scenarios (for instance
the Xilinx work Andreas mentioned, where as I understand it they
have one tool that generates the DTB and the FPGA they're trying
to model and so it's easy to be sure the DTB has all the necessary
information).

The fundamental issue is that a DTB is basically a data structure
controlled by the kernel -- it contains all the information the
kernel needs to know about how the hardware is configured and
wired up together, where the kernel can't dynamically probe for it.
The information QEMU needs to know to be able to model a board is
related but not the same (and why would the kernel people modify
their device tree files to add information that only QEMU cares
about?) So it sounds nice but in practice it gets very quickly
into a morass of problems I think.

[To take one example, boards with CPUs that support TrustZone
may have devices that appear only in the Secure world, but there's
no way in a device tree to indicate this. The kernel doesn't
care because it only executes in one world and only needs to
know about the devices that exist there.]

-- PMM