Re: [Qemu-devel] [PATCH v2 5/7] qmp: add set-bootindex command

[Markus Armbruster <armbru@redhat.com>]

"Gonglei (Arei)" <arei.gonglei@huawei.com> writes:

> Hi, Markus
>
>> >> >> Subject: Re: [Qemu-devel] [PATCH v2 5/7] qmp: add set-bootindex
>> command
>> >> >>
>> >> >> On Fri, Jul 25, 2014 at 02:52:50PM +0800, arei.gonglei@huawei.com
>> wrote:
>> >> >> > From: Gonglei <arei.gonglei@huawei.com>
>> >> >> >
>> >> >> > Adds "set-bootindex id=xx,bootindex=xx,suffix=xx" QMP command.
>> >> >> >
>> >> >> > Example QMP command:
>> >> >> > -> { "execute": "set-bootindex", "arguments": { "id": "ide0-0-1",
>> >> >> > "bootindex":
>> >> >> 1, "suffix": "/disk@0"}}
>> >> >> > <- { "return": {} }
>> >> >> >
>> >> >> > Signed-off-by: Gonglei <arei.gonglei@huawei.com>
>> >> >> > Signed-off-by: Chenliang <chenliang88@huawei.com>
>> >> >> > ---
>> >> >> >  qapi-schema.json | 16 ++++++++++++++++
>> >> >> >  qmp-commands.hx  | 24 ++++++++++++++++++++++++
>> >> >> >  qmp.c            | 17 +++++++++++++++++
>> >> >> >  3 files changed, 57 insertions(+)
>> >> >> >
>> >> >> > diff --git a/qapi-schema.json b/qapi-schema.json
>> >> >> > index b11aad2..a9ef0be 100644
>> >> >> > --- a/qapi-schema.json
>> >> >> > +++ b/qapi-schema.json
>> >> >> > @@ -1704,6 +1704,22 @@
>> >> >> >  { 'command': 'device_del', 'data': {'id': 'str'} }
>> >> >> >
>> >> >> >  ##
>> >> >> > +# @set-bootindex:
>> >> >> > +#
>> >> >> > +# set bootindex of a devcie
>> >> >> > +#
>> >> >> > +# @id: the name of the device
>> >> >> > +# @bootindex: the bootindex of the device
>> >> >> > +# @suffix: #optional a suffix of the device
>> >> >> > +#
>> >> >> > +# Returns: Nothing on success
>> >> >> > +#          If @id is not a valid device, DeviceNotFound
>> >> >> > +#
>> >> >> > +# Since: 2.2
>> >> >> > +##
>> >> >> > +{ 'command': 'set-bootindex', 'data': {'id': 'str', 'bootindex':
>> >> >> > int', '*suffix':
>> >> >> 'str'} }
>> >> >> > +
>> >> >> > +##
>> >> >>
>> >> >> I wonder if we could simply use qom-set for that. How many devices
>> >> >> actually support having multiple bootindex entries with different
>> >> >> suffixes?
>> >> >>
>> >> > AFAICT, the floppy device support two bootindex with different suffixes.
>> >>
>> >> Block device models commonly a single block device.  By convention,
>> >> property "drive" is the backend, and property "bootindex" the bootindex,
>> >> if the device model supports that.
>> >>
>> >> The device ID suffices to identify a block device for such device
>> >> models.
>> >>
>> >> The floppy device model is an exception.  It folds multiple real-world
>> >> objects into one: the controller and the actual drives.  Have a look at
>> >> -device isa-fdc,help:
>> >>
>> >>     isa-fdc.iobase=uint32
>> >>     isa-fdc.irq=uint32
>> >>     isa-fdc.dma=uint32
>> >>     isa-fdc.driveA=drive
>> >>     isa-fdc.driveB=drive
>> >>     isa-fdc.bootindexA=int32
>> >>     isa-fdc.bootindexB=int32
>> >>     isa-fdc.check_media_rate=on/off
>> >>
>> >> The properties ending with 'A' or 'B' apply to the first and the second
>> >> drive, the others to the controller.
>> >>
>> >> Unfortunately, this means the device ID by itself doesn't identify the
>> >> floppy device.
>> >>
>> > Yes.
>> >
>> >> Arguably, this is lousy modeling --- we really should model separate
>> >> real-world objects as separate objects.  But making floppies pretty (or
>> >> even sane) isn't anyone's priority nowadays.
>> >>
>> > Hmm... Agreed.
>> >
>> >> Since qom-set works on *properties*, I can't see why it couldn't be used
>> >> for changing bootindexes.  There is no ambiguity even with floppy.
>> >
>> > Sorry?
>> >
>> >> You either set bootindexA or bootindexB.  No need to fuzz around with
>> >> suffixes.  Or am I missing something?
>> >
>> > Your mean that we just need to think about change one bootindex? Either
>> > set bootindexA or bootindexB, do not need pass suffix for identifying?
>> 
>> Eduardo suggested to think about using qom-set to update the bootindex.
>> 
>> Could look like
>> 
>>     { "execute": "qom-set",
>>       "arguments": { "path": "/machine/unattached/device[15]",
>>                      "property": "bootindexA", "value": 1 } }
>> 
>> Demonstrates an existing, well-defined way to identify the bootindex to
>> change.  Do we really want to invent another one, based on suffix?
>> 
>> The value of "path" is the QOM path.  I can't remember offhand how to go
>> from qdev ID to QOM path.  Onboard devices like isa-fdc don't have one
>> anyway.
>> 
>> I also don't remember whether there's a nicer QOM path than this one.
>
> Thanks for your explaining. TBH I haven't used "qom-set" before, so I

Few people have :)

> don't know what your mean (like Eduardo, sorry again). 
>
> But now, I have a look at the implement of "qom-set" command, and 
> I find the command is just change a device's property value, do not have
> any other logic. For my case, we really change value of the global
> fw_boot_order list,
> which the device's bootindex property worked for actually. Only in this way,
> we can attach our original target.

Why can't we delay the logic to the next reboot?  Let me explain.

Current code starts with empty fw_boot_order, then lets device realize
add to it.  Unplugging a device leaves a dangling DeviceState pointer in
the list (I think).

Could we instead build, use and free the list during reboot?  That way,
qom-set of bootindex affects the next reboot without additional logic.