[Qemu-block] [PATCH v2 for-2.4 0/3] block: Improve warnings for doubly-connected drives

[Peter Maydell]

This patchset attempts to improve the warning and error messages for
bad user command lines that attempt to connect a drive up to two
devices. The motivation here is patch #4, which changes the default
interface for the virt board to virtio. That will break some existing
command lines which forgot to specify if=none, and so I would like
us to at least diagnose that user error in a helpful way that points
the user towards adding the missing if=none.

Version 2 reduces scope, because it turns out that it is harder than
we thought to identify whether a use of an if=something drive is the
auto-plugging or a manual wiring up to a device. So all we do here
is improve the error messages for two situations which were already
errors but with rather cryptic messages:

    (1) Drive specified as to be auto-connected and also manually connected
    (and the board does handle this if= type):
    
      qemu-system-x86_64 -nodefaults -display none \
         -drive if=scsi,file=tmp.qcow2,id=foo -device ide-hd,drive=foo
    
    Previously:
      qemu-system-x86_64: -device ide-hd,drive=foo: Property 'ide-hd.drive'
      can't take value 'foo', it's in use
    
    Now:
      qemu-system-x86_64: -device ide-hd,drive=foo: Drive 'foo' is already in
      use because it has been automatically connected to another device (did
      you need 'if=none' in the drive options?)
    
    (2) Drive specified to be manually connected in two different ways:
    
      qemu-system-x86_64 -nodefaults -display none \
       -drive if=none,file=tmp.qcow2,id=foo -device ide-hd,drive=foo \
       -device ide-hd,drive=foo
    
    Previously:
      qemu-system-x86_64: -device ide-hd,drive=foo: Property 'ide-hd.drive'
      can't take value 'foo', it's in use
    
    Now:
      qemu-system-x86_64: -device ide-hd,drive=foo: Drive 'foo' is already in
      use by another device

(We'll also produce message 1 in the oddball case where the user creates
a drive if=something-not-handled-by-machine and then wires it up manually
to two different devices; in this case their command line is doubly broken
and if they use if=none as suggested by message 1 they'll then get message 2
and can fix their own double-usage...)

Changes v1->v2:
 * drop "warn if an if=<something> drive was also connected manually" patch
 * change implementation of "improve error message" patch


Peter Maydell (3):
  qdev-properties-system: Change set_pointer's parse callback to use
    Error
  qdev-properties-system: Improve error message for drive assignment
    conflict
  hw/arm/virt: Make block devices default to virtio

 hw/arm/virt.c                    |  2 ++
 hw/core/qdev-properties-system.c | 42 +++++++++++++++++++++++++++-------------
 2 files changed, 31 insertions(+), 13 deletions(-)

-- 
1.9.1