[Qemu-devel] How to reliably start a bare QEMU target to query capabilities via QMP

[Daniel P. Berrange]

I've been adapting libvirt to use to the various new QMP commands to
query QEMU's capabilities, instead of the hated -help parsing. Obviously
the critical part of this is being able to reliably start a bare QEMU
process with no actual guest OS configured (no disks, no kernel, etc)
and talk to its monitor. I hadn't anticipated problems since I only
tested with x86_64 / i386 most of the time, which work well in this
respect. Currently I am doing

  # $QEMU_BINARY -S \
         -no-user-config \
         -nodefconfig \
         -nodefaults \
         -nographic \
         -qmp stdio


This works for about 50% of the QEMU targets:

    qemu-system-alpha
    qemu-system-i386
    qemu-system-lm32
    qemu-system-ppc
    qemu-system-ppc64
    qemu-system-s390x
    qemu-system-sparc
    qemu-system-sparc64
    qemu-system-x86_64
    qemu-system-xtensa
    qemu-system-xtensaeb

 but fails for the other 50% of targets:

    qemu-system-arm
    qemu-system-cris
    qemu-system-m68k
    qemu-system-mips
    qemu-system-mips64
    qemu-system-mips64el
    qemu-system-mipsel
    qemu-system-or32
    qemu-system-ppcemb
    qemu-system-sh4
    qemu-system-sh4eb
    qemu-system-unicore32


With the failing targets i see the following kinds of errors:


 $ qemu-system-arm -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 Kernel image must be specified

 $ qemu-system-cris -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 Kernel image must be specified

 $ qemu-system-m68k -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 Kernel image must be specified

 $ qemu-system-mips -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait qemu: Could not load MIPS bios 'mips_bios.bin', and 
no -kernel argument was specified

 $ qemu-system-mipsel -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 qemu: Could not load MIPS bios 'mipsel_bios.bin', and no -kernel argument was 
specified

 $ qemu-system-or32 -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 Can't create serial device, empty char device

 $ qemu-system-ppcemb -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 Unable to find PowerPC CPU definition

 $ qemu-system-sh4 -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 Initializing CPU
 Allocating ROM
 Allocating SDRAM 1
 Allocating SDRAM 2
 shix_init: load BIOS 'shix_bios.bin'
 ret=-1
 qemu: could not load SHIX bios 'shix_bios.bin'

 $ qemu-system-sh4eb -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 Initializing CPU
 Allocating ROM
 Allocating SDRAM 1
 Allocating SDRAM 2
 shix_init: load BIOS 'shix_bios.bin'
 ret=-1
 qemu: could not load SHIX bios 'shix_bios.bin'

 $ qemu-system-unicore32 -S -nodefconfig -nodefaults -nographic -qmp 
unix:/tmp/foo,server,nowait
 qemu-system-unicore32: /home/berrange/src/virt/qemu/hw/unicore32/../puv3.c:81: 
puv3_load_kernel: Assertion `kernel_filename != ((void *)0)' failed.
 Aborted


Can we "fix" them in some way to not require the kernel ?

Do we have to go down the route of adding some sort of "-no-vm" flag to
explicitly say we don't care about any of the VM setup parts ? This would
be more like what my old patches did which mapped monitors commands
onto '-query-XXXXX' command line args, bypassing VM setup, but still
using normal QMP monitor interaction.

Any other suggestions on how to reliably get a QMP monitor to a target,
without any VM config. ?

Daniel
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