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Re: About creating machines on the command line

From: Luc Michel
Subject: Re: About creating machines on the command line
Date: Fri, 5 Feb 2021 11:43:38 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1

On 2/3/21 6:09 PM, Alexander Graf 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.

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?
Yes, after some discussions with the community, we are now working on improving QMP to achieve this. We first started with the idea of the command line because it seems to be the place where we had "almost" everything we needed already. In either cases we are planning to use a front-end script to go from e.g. a DTB to whatever QEMU interface we will end up using.

You could then have machine descriptions in a script, which could be generated by the SoC customization tools.
Yes, most likely a DTB in our case.

In combination with plugin support for platform devices, that should allow pretty much any customization you would need to happen, no?
I'm not sure what you mean by "plugin support for platform devices". If you refer to the current QEMU plugin API, it's pretty much limited to TCG. If you suggest this API could be enhanced to allow for custom devices to be added by plugins, for now we are planning to use only existing device models. It's more a matter of having flexibility on connections between the devices than on the devices themselves (but yes that would be a nice next step).




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:

   - 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.

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

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

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

   - Add command line options to connect GPIOs and clocks.

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.

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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