Re: [Qemu-devel] [qemu devel] disable shared memory is not available with this QEMU binary

[Marcel Apfelbaum]

On 03/31/2015 05:21 PM, Tony Krowiak wrote:
> Commit 49d2e648e8087d154d8bf8b91f27c8e05e79d5a6 removed the QemuOptDesc 
> elements from the
> *desc* field of the *qemu_machine_opts *array defined in vl.c.  Since applying 
> that patch to qemu
> on my system, I can not start a guest from libvirt when certain machine options 
> are configured
> for the guest domain.  For example, if I configure the following for my guest 
> domain:
>
>      <memoryBacking>
>          ...
>          <nosharepages>
>          ...
>      </memoryBacking>
>
> I get the following libvirt error when I try to start the guest:
>
>      error: unsupported configuration: disable shared memory is not available 
> with this QEMU binary
>
> The *nosharepages *element generates the *-machine* option *mem-merge=off* on 
> the QEMU command line.  The error is
> thrown by libvirt because the QMP *query-command-line-options* command does not 
> return *mem-merge* in the machine
> options parameter list.  In fact, if I issue the *query-command-line-options* 
> command via virsh as follows:
>
>      virsh qemu-monitor-command guest_c2aa '{ "execute": "query-command-line-options", "arguments": 
> { "option": "machine" } }'
Hi Tony,
Thank you for finding this bug.

> No machine option parameters are returned:
>
> {"return":[{"parameters":[],"option":"machine"}],"id":"libvirt-11"}
Indeed, we have a problem here.

This is the first object for which QemuOps are defined per
sub-type and are not global (if you don't take "object" under consideration).
I saw others as well, like netdev, but I am not sure what happens there.

Once the QemuOpts are parsed, the only place we can find those options
is the machine object itself (as QOM properties).

I see a few options here:
1. Add a feature to QemuOpts: "Look for options in QOM properties of this obj"
2. Add a callback to QEMU opts that supplies the options (have machine supply 
the callback)
3. Have the machine object fill in the corresponding QemuOpts on init.

Any thoughts?
Thanks,
Marcel



> I took a look at the *qmp_query_command_line_options* function in 
> *util/qemu-config.c*.  The function derives the
> option parameters to return with the the query response from the QemuOptDesc 
> elements contained in the
> *desc* field of the *qemu_machine_opts *array defined in vl.c.  It appears that 
> removing the
> QemuOptDesc elements broke the *qmp_query_command_line_options* function.  If I 
> restore the QemuOptDesc
> elements removed by commit 49d2e648e8087d154d8bf8b91f27c8e05e79d5a6, I can 
> start the guest with
> *nosharepages* configured.
>
> It would appear that a bug was introduced with commit 
> 49d2e648e8087d154d8bf8b91f27c8e05e79d5a6,
> what say you?