Re: [PATCH 0/9] hw/sysbus/platform-bus: Introduce TYPE_DYNAMIC_SYS_BUS_DEVICE

[Peter Maydell <peter.maydell@xxxxxxxxxx>]

On Tue, 28 Jan 2025 at 10:42, Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
>
> On Sat, Jan 25, 2025 at 07:13:34PM +0100, Philippe Mathieu-Daudé wrote:
> > Some SysBus devices can optionally be dynamically plugged onto
> > the sysbus-platform-bus (then virtual guests are aware of
> > mmio mapping and IRQs via device tree / ACPI rules).
>
> Do we have some sane way to have user-pluggable sysbus devices on arm?

The answer in a general sense is "no, because user pluggable
sysbus is a weird idea". "sysbus" means "it's wired into a
specific bit of the memory map and to specific IRQs, and whoever
does that needs to know what IRQs and bits of memory are usable,
and the guest OS needs to know it's there". "user-pluggable" means
"it's all automatic and the guest can just do some kind of
probing for what is or isn't present". All the platform bus stuff
is a nasty mess that's working around the things people want
to plug in not being clean devices on probeable buses :-(
And the platform bus is only supported on the "virt" board,
because that's the only one where QEMU is generating its
own dtb or ACPI tables where it can tell the guest "hey,
there's some device here".

-- PMM