Re: [PATCH qemu.git 0/1] hw/arm/virt: add 2x sp804 timer

[Peter Maydell]

On Thu, 8 Dec 2022 at 16:59, Axel Heider <axelheider@gmx.de> wrote:
>
> Peter,
>
> >> This patch adds timer peripherals to the arm-virt machine.>>
> > Is there a reason you can't use the CPU's built-in generic timer
> > device ? That is what typical guest code does on this system.
> > I'm a bit reluctant to add more devices to the virt board
> > because over time it gradually gets increasingly complicated,
> > and every new device model we expose to the guest is another
> > thing that's part of the security attack surface for guest
> > code trying to escape from a KVM VM.
>
> For the seL4 specific case, this is currently not possible in
> the standard configuration. It's only exposed for a special
> debug and benchmarking configuration.

It's not clear to me what you mean here -- the generic
timer in the CPU exists in all configurations, so there
should be no obstacle to seL4 using it.

> The catch we have here is, that the virt machine is a nice
> generic ARM (and RISC-V) machine for OS testing purposes also,
> but it sometimes lacks things (see my other patched for the
> UART). So, I wonder what would be the best option to continue
> here. Should we consider defining another generic machine
> profile that is more suited for the system emulation use case.
> This is what OS developer could use then. Or could the virt
> machine get some config parameters to customize it further.
> So the "Machine-specific options" would  support a "sp804=on"
> that would add two timer peripherals then?
>
> The really cool customization option would be passing a DTB
> to QEMU that describes exactly what "virt" machine is to be
> emulated.

This is a firm "no" -- it sounds on the surface like a good
idea but it doesn't actually work in practice -- DTB files
don't provide enough info to be able to build a board from,
except in some specific restricted situations like the Xilinx one.

-- PMM