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Re: [RFC PATCH v2 00/16] Initial support for machine creation via QMP


> On 13 Oct 2021, at 00:16, Alistair Francis <alistair23@xxxxxxxxx> wrote:
> On Thu, Sep 23, 2021 at 2:22 AM Damien Hedde <damien.hedde@xxxxxxxxxxxxx> 
> wrote:
>> Hi,
>> The goal of this work is to bring dynamic machine creation to QEMU:
>> we want to setup a machine without compiling a specific machine C
>> code. It would ease supporting highly configurable platforms (for
>> example resulting from an automated design flow). The requirements
>> for such configuration include begin able to specify the number of
>> cores, available peripherals, emmory mapping, IRQ mapping, etc.
>> This series focuses on the first step: populating a machine with
>> devices during its creation. We propose patches to support this
>> using QMP commands. This is a working set of patches and improves
>> over the earlier rfc (posted in May):
>> https://lists.gnu.org/archive/html/qemu-devel/2021-05/msg03706.html
>> Although it is working and could be merged, it is tag as an RFC:
>> we probably need to discuss the conditions for allowing a device to
>> be created at an early stage. Patches 6, 10 and 13, 15 and 16 depend
>> on such conditions and are subject to change. Other patches are
>> unrelated to this point.
>> We address several issues in this series. They are detailed below.
>> ## 1. Stoping QEMU to populate the machine with devices
>> QEMU goes through several steps (called _machine phases_) when
>> creating the machine: 'no-machine', 'machine-created',
>> 'accel-created', 'initialized', and finally 'ready'. At 'ready'
>> phase, QEMU is ready to start (see Paolo's page
>> https://wiki.qemu.org/User:Paolo_Bonzini/Machine_init_sequence for
>> more details).
>> Using the -preconfig CLI option, QEMU can be stopped today during
>> the 'accel-created' phase. Then the 'x-exit-preconfig' QMP command
>> triggers QEMU moving forwards to the completion of the machine
>> creation ('ready' phase).
>> The devices are created during the 'initialized' phase.
>> In this phase the machine init() method has been executed and thus
>> machine properties have been handled. Although the sysbus exists and
>> the machine may have been populated by the init(),
>> _machine_init_done_ notifiers have not been called yet. At this point
>> we can add more devices to a machine.
>> We propose to add 2 QMP commands:
>> + The 'query-machine-phase' command would return the current machine
>>  phase.
>> + The 'x-machine-init' command would advance the machine phase to
>>  'initialized'. 'x-exit-preconfig' could then still be used to
>>  advance to the last phase.
>> ## 2. Adding devices
>> Right now, the user can create devices in 2 ways: using '-device' CLI
>> option or 'device_add' QMP command. Both are executed after the
>> machine is ready: such devices are hot-plugged. We propose to allow
>> 'device_add' QMP command to be used during the 'initialized' phase.
>> In this series, we keep the constraint that the device must be
>> 'user-creatable' (this is a device class flag). We do not see any
>> reason why a device the user can hot-plug could not be created at an
>> earlier stage.
>> This part is still RFC because, as Peter mentioned it (in this thread
>> https://lists.gnu.org/archive/html/qemu-devel/2021-08/msg01933.html),
>> we may want additional or distinct conditions for:
>> + device we can hot-plug
>> + device we can add in '-preconfig' (cold-plug)
>> We are open to suggestions. We could for example add a
>> 'preconfig-creatable' or 'init-creatable' flag to device class, which
>> can identify a set of devices we can create this way.
>> The main addition is how we handle the case of sysbus devices. Sysbus
>> devices are particular because unlike, for example, pci devices, you
>> have to manually handle the memory mapping and interrupts wiring. So
>> right now, a sysbus device is dynamically creatable (using -device
>> CLI option or device_add QMP command) only if:
>> + it is 'user_creatable' (like any other device),
>> + and it is in the current machine sysbus device allow list.
>> In this series, we propose to relax the second constraint during the
>> earlier phases of machine creation so that when using -preconfig we
>> can create any 'user-creatable' sysbus device. When the machine
>> progresses to the 'ready' phase, sysbus devices creation will come
>> back to the legacy behavior: it will be possible only based on the
>> per-machine authorization basis.
>> For sysbus devices, wiring interrupts is not a problem as we can use
>> the 'qom-set' QMP command, but memory mapping is.
>> ## 3. Memory mapping
>> There is no point allowing the creation sysbus devices if we cannot
>> map them onto the memory bus (the 'sysbus').
>> As far as we know, right now, there is no way to add memory mapping
>> for sysbus device using QMP commands. We propose a 'x-sysbus-mmio-map'
>> command to do this. This command would only be allowed during the
>> 'initialized' phase when using -preconfig.
>> ## 4. Working example
>> The last patches of the series add and modify devices in order to
>> build a working machine starting from the 'none' machine.
>> We add a new sysbus device modeling a simple memory (ram or rom). We
>> also set 'user-creatable' flag of some sysbus devices. These are
>> trivial patches, but they depends on the conditions we choose to allow
>> creating devices with -preconfig. Therefore, there is really no need
>> to review them until we settled on the device conditions first.
>> With these devices (memory, ibex_uart, ibex_plic) we can dynamically
>> configure a part (we did not add the timer, but we could) the
>> opentitan machine very easily and run firmwares which demonstrates
>> interrupts and memory-mapping are working.
>> We use the existing qmp-shell script to issue machine devices
>> from a qmp commands script file which contains qmp commands listed in
>> a file.
>> The following qmp commands add some memories, an interrupt controller
>> and an uart with an interrupt.
>> cat > opentitan.qmp <<EOF
>> x-machine-init
>> # ROM 0x00008000
>> device_add        driver=sysbus-memory id=rom size=0x4000 readonly=true
>> x-sysbus-mmio-map device=rom addr=32768
>> # FLASH 0x20000000
>> device_add        driver=sysbus-memory id=flash size=0x80000 readonly=true
>> x-sysbus-mmio-map device=flash addr=536870912
>> # RAM 0x10000000
>> device_add        driver=sysbus-memory id=ram size=0x10000
>> x-sysbus-mmio-map device=ram addr=268435456
>> # PLIC 0x41010000
>> device_add        driver=ibex-plic id=plic
>> x-sysbus-mmio-map device=plic addr=1090584576
>> # UART 0x40000000
>> device_add        driver=ibex-uart id=uart chardev=serial0
>> x-sysbus-mmio-map device=uart addr=1073741824
>> qom-set path=uart property=sysbus-irq[1] value=plic/unnamed-gpio-in[2]
>> x-exit-preconfig
>> EOF
>> We've put the opentitan.qmp and a firmware opentitan-echo.elf here
>> (among some other qmp machine files we are working on):
>> https://github.com/GreenSocs/qemu-qmp-machines
> I am unable to access this repo, maybe it's not public?
> Alistair



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