Re: [Qemu-devel] [PATCH] boards: add a 'none' machine type to all platforms

[Anthony Liguori <aliguori@us.ibm.com>]

Eric Blake <eblake@redhat.com> writes:

> On 08/22/2012 02:24 PM, Anthony Liguori wrote:
>> This allows any QEMU binary to be executed with:
>> 
>>   $QEMU_BINARY -qmp stdio
>
> Don't you mean:
>
> $QEMU_BINARY -M none -qmp stdio

I did, thanks.

>
> I agree with including this in 1.2, as otherwise your new query-target
> and other commands are incomplete (that is, this is a 'bug fix' of
> rounding out a feature already promised at hard freeze, and not a new
> feature on its own).
>
>> +static QEMUMachine machine_none = {
>> +    .name = "none",
>> +    .desc = "empty machine",
>> +    .init = machine_none_init,
>> +    .max_cpus = 0,
>> +};
>
> I guess libvirt just blindly tries '-S -M none'; if it works, we must be
> talking to new enough qemu (and all the other QMP commands that we want
> to probe are then immediately available);

Correct.  '-M none' will fail with a non-zero exit status in all old
versions of QEMU.

-S isn't really needed FWIW but it certainly doesn't hurt.  There aren't
any VCPUs created with -M none so strictly speaking, -S doesn't do
anything.

No ram is allocated with -M none either which is another nice touch
(less resource usage).

> if it doesn't work, then we
> must be talking to older qemu and can fall back to -help scraping (since
> older versions won't be further modifying their -help output now that
> they are released). 

Correct.

Regards,

Anthony Liguori

> I like the idea, although I'm not familiar enough
> with this part of the code to know if my review counts for anything:
>
> Reviewed-by: Eric Blake <eblake@redhat.com>
>
> -- 
> Eric Blake   eblake@redhat.com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org