[libvirt] [PATCH] qemu-capabilities: Fix query-cpu-model-expansion on s390 with older kernel

[Jason J. Herne]

On 01/13/2017 05:04 AM, Jiri Denemark wrote:
> On Thu, Jan 12, 2017 at 11:18:11 -0500, Collin L. Walling wrote:
>> When running on s390 with a kernel that does not support cpu model checking and
>> with a Qemu new enough to support query-cpu-model-expansion, the gathering of qemu
>> capabilities will fail. Qemu responds to the query-cpu-model-expansion qmp
>> command with an error because the needed kernel ioct does not exist. When this
>> happens a guest cannot even be defined due to missing qemu capabilities data.
>>
>> This patch fixes the problem by silently ignoring generic errors stemming from
>> calls to query-cpu-model-expansion.
>>
>> Reported-by: Farhan Ali <alifm at linux.vnet.ibm.com>
>> Signed-off-by: Collin L. Walling <walling at linux.vnet.ibm.com>
>> Signed-off-by: Jason J. Herne <jjherne at linux.vnet.ibm.com>
>> ---
>>  src/qemu/qemu_monitor_json.c | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/src/qemu/qemu_monitor_json.c b/src/qemu/qemu_monitor_json.c
>> index e767437..1662749 100644
>> --- a/src/qemu/qemu_monitor_json.c
>> +++ b/src/qemu/qemu_monitor_json.c
>> @@ -5041,6 +5041,15 @@ qemuMonitorJSONGetCPUModelExpansion(qemuMonitorPtr mon,
>>      if (qemuMonitorJSONCommand(mon, cmd, &reply) < 0)
>>          goto cleanup;
>>
>> +    /* Some QEMU architectures have the query-cpu-model-expansion
>> +     * command, but return 'GenericError' instead of simply omitting
>> +     * the command entirely.
>> +     */
>
> Hmm, this comment says something a bit different than the commit
> message. I hope the issue described by this comment was fixed in QEMU
> within the same release the query-cpu-model-expansion command was
> introduced. But we need to fix this nevertheless to address what the
> commit message describes.
>

The issue still exists in Qemu. I can work up a patch to handle it. My first
idea is to simply test that the ioctl exists and functions at Qemu
initialization and deregister the query-cpu-model-expansion command in the
event that the ioctl does not exist or fails. Thoughts?
>
> However, we need to do a little bit more than just ignoring this error.
> I'll send a v2 soon.

-- 
-- Jason J. Herne (jjherne at linux.vnet.ibm.com)