Re: command line syntax for connecting a chardev to a CPU

[Markus Armbruster <armbru@redhat.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> Arm CPUs have a "debug communications channel" which on real hardware
> is basically a way to talk to the debugger on the other end of a JTAG
> connection; Linux supports using this as a console. This patchseries:
>  https://patchew.org/QEMU/20240614093026.328271-1-sai.pavan.boddu@amd.com/
> proposes implementing this in QEMU by wiring it up to a QEMU chardev.
>
> I think this is useful (among other things, it lets the user sidestep
> the "where is my UART?" question). But I'm not sure what the right way
> to let the user enable it and pick the chardev on the command line is.
> Do we have any relevant existing precedent?
>
> The patchseries has the CPU look for a chardev by ID, so if the user
> creates a chardev with id=dcc0 the first CPU will use that, if there's
> a chardev with id=dcc1 the second CPU will use that, and so on. I
> don't think we really want to make some ID string values be magic,

Neither do I.

> but maybe we do that already somewhere, and so it's OK to do here?

I'm not aware of such existing (ab)use of chardev IDs.

> I thought also of having the CPU take a chardev property, but then the
> question is how to specify that on the command line. AFAICT the -cpu
> option (a) requires a CPU type first, which is a pain for cases where
> otherwise the user has no need to care about the exact type of CPU
> because the machine model creates the right one for them, and (b) for
> the key=value properties in a -cpu option string it will set the same
> property value for every CPU in the system (which obviously isn't what
> we want for this chardev).

Looks like an instance of the old "how to set properties of onboard
devices" problem.  Still no good solution.

> We could make it a machine property (so you would say eg
>  -M xlnx-zcu102,dcc0=mychardev -chardev stdio,id=mychardev)
> but then that would require plumbing code in every machine model to
> create the property and set the value on the right CPU.

Machine properties that are aliases of the to onboard device properties
we want to set is a solution we used in places.  Requires plumbing, as
you wrote.

> Do we have a neat way to specify per-cpu CPU properties that I'm missing?

I'm not aware of a better solution.