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Re: About creating machines on the command line

From: John Snow
Subject: Re: About creating machines on the command line
Date: Thu, 4 Feb 2021 15:29:02 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 2/3/21 12:09 PM, graf--- via wrote:

On 03.02.21 17:55, Philippe Mathieu-Daudé wrote:

On 1/11/21 3:50 PM, Luc Michel wrote:

We would like to work on improving QEMU to be able to create custom
machines from the command line. The goal here is to get feedback from
the community and shape the future developments.

Cc'ing John who is working on command line, and some developers from
the "inter-VM device emulation interface" call.

I have a bunch of extremely naive questions about what it would take to generalize this kind of interface; my familiarity is largely with x86 and so I actually just have a series of very basic questions about the scope of this work.

The use case mainly comes from people working with tools to customize
their designs, such as SiFive Core Designer
(https://scs.sifive.com/core-designer). This kind of tools may allow
creation or customization of a whole SoC, from the number of cores, to
the memory and IRQ mapping of peripherals etc.

The ultimate goal would be to be able to create any kind of machine on
the command line. However we are aware that this is a substantial amount
of changes in QEMU.

Is the command line really the right abstraction level here? Wouldn't it make more sense to have a QOM / <scripting language> bridge that allows you to create and connect QOM objects using for example Python?

You could then have machine descriptions in a script, which could be generated by the SoC customization tools.

In combination with plugin support for platform devices, that should allow pretty much any customization you would need to happen, no?


Right, I am just reading this proposal as "We want an interface language that allows us to construct more arbitrary configurations."

In this case, something QMP/QOM-based would be the right place to do it and not the CLI as it exists today. Small details in the grand scheme of things the kind of internal modeling cleanup this idea seems to imply.

In its current state, QEMU allows for very limited customization of
existing machines on the command line. We identified the following
limitations (feel free to add to the list):

   - Most devices are not user creatable. Moreover, sysbus devices must
be explicitly allowed by a machine to be creatable through `-device`,

   - Memory regions cannot be created on the command line,

   - Device MMIO regions cannot be mapped on a bus from the command line,

   - GPIOs and clocks cannot be wired from the command line,

   - CPUs are not sysbus devices (and not user-creatable). They need
special care when creating them regarding system reset. Not being on a
bus means that they must be reset manually on system reset. This is done
in machines by registering a QEMU reset handler.

   - Machine specific boot code is usually hard-coded into the machine
itself.  Some architectures (e.g. ARM) do factorize bootloader related
code, but there is no standard way of doing that in QEMU.

We don't want to address all those limitations at once. We plan to start
with the following scenario:

It does sound like a staggering level of work.

   - Start with a base machine that would handle CPU creation and
bootloader stuff. Note that the "none" machine is probably not
sufficient in its current shape. It does allow only one CPU and
obviously does not handle the boot process.

I suppose this implies a type of machine that doesn't have migration compatibility. e.g. your "blank" might be different from someone else's "blank".

Or at least, not migration compatibility in its current form. Some thought might need to be put into how to negotiate migration compatibility between two instances of QEMU; it sounds like a hard problem.

(Jokingly, I'd want to call a machine like this "Apple Pie", but that would probably be too confusing for both Apple and Raspberry Pi enthusiasts.)

   - Allow for this machine every sysbus devices we want to be user
command-line creatable (and mark them user_creatable if needed)

Is it even possible to enumerate, for every sysbus device in QEMU, what the interface requirements are?

e.g. I assume most need a memory region, or maybe an IO region, or maybe an IRQ resource of some kind, or ...

doing this kind of dynamic bringup requires a lot of configuration data that is at-present just hardcoded absolutely everywhere.

   - Add command line options to create memory regions (probably ram ones
at first)

Naive question: what does adding a memory region mean to you in this context?

For whom (which device, spec, standard, instruction set, etc?) are these regions tangible abstractions?

   - Add command line options to map a memory region (including sysbus
device MMIO regions) onto another (memory_region_add_subregion)

More naive questions:

The assumption here being that sysbus devices can be modeled using their own private memory addressing namespace; and we configure them by mapping that space onto the machine's physical address space.

Instead of, say, making the memory address of that device a property of the device.


   - Add command line options to connect GPIOs and clocks.

Similar to elsewhere, I wonder how much work it is to generalize this kind of pluggability so that all existing clocks and GPIO devices can be modeled in the same way.

(I say this meaning: Wow, I have no idea. I know a lot of the ones we have are hard-coded. What does a configuration interface for this look like?)

This would hopefully allow for simple machines creation. We would then
be able to use either the command line or the `-readconfig` option to
create the machine.

Note that we are not planning to use QMP/HMP for now. From our
understanding, a `device_add` request is always considered as hot-plug,
which is not what we want here.

We're working our way towards being able to use QMP to define machines prior to execution start in order to have a QMP-only interaction with QEMU possible.

You likely want to invest in that kind of a strategy where you define structures that describe the machines you want to build.

Please tell us what do you think about this plan. Any feedback is
appreciated.  Then we can discuss the details of how to do this properly.


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