Re: About creating machines on the command line

[Philippe Mathieu-Daudé]

On 1/11/21 3:50 PM, Luc Michel wrote:
> Hi,
> 
> 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.
https://www.mail-archive.com/qemu-devel@nongnu.org/msg723252.html

> 
> 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.
> 
> 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.
> Thanks!
> 
> -- 
> Luc
>