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

[Anthony Liguori <anthony@codemonkey.ws>]

"Daniel P. Berrange" <berrange@redhat.com> writes:

> On Tue, Dec 04, 2012 at 01:13:46PM -0600, Anthony Liguori wrote:
>> "Daniel P. Berrange" <berrange@redhat.com> writes:
>> 
>> > On Tue, Dec 04, 2012 at 03:42:32PM +0100, Ján Tomko wrote:
>> >> On 12/04/12 12:46, Luiz Capitulino wrote:
>> >> > On Mon, 03 Dec 2012 16:55:35 +0100
>> >> > Ján Tomko <jtomko@redhat.com> wrote:
>> >> > 
>> >> >> Hello,
>> >> >>
>> >> >> is there a way to check if QEMU was compiled with --enable-seccomp via QMP?
>> >> > 
>> >> > Not that I'm aware of. Could you describe your use-case?
>> >> 
>> >> It's for libvirt. The detection is broken since the switch from parsing
>> >> -help output to QMP and I wanted to fix it.
>> >> 
>> >> Assuming it's supported if we do capabilities detection via QMP (since
>> >> libvirt 1.0.0 and QEMU 1.2) would work except for this case:
>> >> If seccomp sandbox was requested in /etc/libvirt/qemu.conf, but it was
>> >> compiled out from qemu, libvirt would try to run QEMU with -sandbox on
>> >> instead of printing an error earlier.
>> >
>> > 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).

Regards,

Anthony Liguori

>
> Regards,
> Daniel
> -- 
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