Re: [Qemu-devel] detecting seccomp sandbox capability via QMP

[Anthony Liguori]

"Daniel P. Berrange" <address@hidden> writes:

> On Tue, Dec 04, 2012 at 03:44:54PM -0600, Anthony Liguori wrote:
>> "Daniel P. Berrange" <address@hidden> writes:
>> 
>> > On Tue, Dec 04, 2012 at 01:13:46PM -0600, Anthony Liguori wrote:
>> >> "Daniel P. Berrange" <address@hidden> writes:
>> >> 
>> >> >
>> >> > In the absence of any way to detect it via QMP, libvirt should fallback
>> >> > to hardcoding it based on the version number. This presumes that QEMU 
>> >> > was
>> >> > built with it enabled in configure, but we've no other option for 
>> >> > current
>> >> > released 1.2/1.3 versions.
>> >> 
>> >> echo quit | qemu -machine none -S -monitor stdio -vnc none -sandbox on
>> >> 
>> >> A non-zero execute means QEMU doesn't support the option.  This will
>> >> work for any new command line option introduction and can be considered
>> >> a "supported" way of probing for whether options are supported.
>> >
>> > One of the significant benefits to libvirt of the QMP based feature
>> > detection, was that we no longer have to invoke QEMU multiple times
>> > to query different data. I don't want to regress in this regard,
>> > because invoking QEMU many times has a noticable performance impact
>> > for some applications eg virt-sandbox were even 100ms delays are
>> > relevant.  So while what you describe does work, I don't think it
>> > is a satisfactory approach for libvirt.
>> 
>> Okay, so in terms of what exists today, I don't have a better option.
>> But we could add:
>> 
>> { 'enum': 'ConfigEntryType',
>>   'data': [ 'number', 'string', 'bool', 'size' ] }
>> 
>> { 'type': 'ConfigEntry',
>>   'data': { 'name': 'str', 'type': 'ConfigEntryType' } }
>> 
>> { 'type': 'ConfigSection',
>>   'data': { 'name': 'str', 'fields': [ 'ConfigEntry' ] } }
>> 
>> { 'command': 'query-config-schema',
>>   'returns': [ 'ConfigSection' ] }


>> 
>> This technically introspects config sections but obviously could be used
>> to detect the availability of -sandbox.
>> 
>> If it's useful, I can take a quick swing at implementing (or someone
>> else certainly could).
>
> I'm not sure I entirely understand what information a 'ConfigSection'
> would represent. By config here, do you mean any command line argument
> or something else ?

We no longer should be adding command line arguments that don't use
QemuOpts and have a equivalent -readconfig syntax.  We could even
eliminate new options and do something like:

qemu -conf sandbox:enable=on

But that's not user friendly so we'll stick with adding higher level
options like -sandbox.

So what I'm proposing is to introspection on what -readconfig supports
and then from that, you can infer when new higher level command line
arguments are added.

>  Could you give a short example of the actual JSON
> you envisage returning for this schema. Your suggestion sounds good,
> but I want to make sure I'm not mis-understanding things :-)

[ { 'name': 'sandbox',
    'fields': [ { 'name': 'enable', 'type': 'bool' } ] },
  { 'name': 'add-fd',
    'fields': [ { 'name': 'fd', 'type': 'number' },
                { 'name': 'set', 'type': 'number' },
                { 'name': 'opaque', 'type': 'str' } ] },
  ...
]

Regards,

Anthony Liguori

>
> Regards,
> Daniel
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