[libvirt] [PATCH] qemu: process: comment on min_guarantee validation

[Cole Robinson]

On 04/19/2016 04:39 AM, Peter Krempa wrote:
> On Mon, Apr 18, 2016 at 19:13:08 -0400, Cole Robinson wrote:
>> Explain why we check it at process startup time, and not parse time
>> where most other XML validation checks are performed
> 
> This is far from being a singular case ...
> 
>> ---
>>  src/qemu/qemu_process.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
>> index c087300..628b4b6 100644
>> --- a/src/qemu/qemu_process.c
>> +++ b/src/qemu/qemu_process.c
>> @@ -4550,6 +4550,8 @@ qemuProcessStartValidate(virQEMUDriverPtr driver,
>>          virDomainDefCheckDuplicateDiskInfo(vm->def) < 0)
> 
> ... this is yet another example of a similar check.
> 
>>          return -1;
>>  
>> +    /* Previously we silently accepted this parameter; we can't reject
>> +       it at parse time without breaking those configs, so check it here */
> 
> I don't think this helps here and implies that other checks are not here
> due to that case. If you want to be explicit I think it warrants a
> separate function with this fact stated in it's comment. If you insist
> on being explicit in the purpose of this check
> 

It's not a matter of 'insist'ing or not; when in this area of the code I saw
the min_guarantee check, which seemed out of place, since there are already
several such checks in the postparse handler.There's no comment explaining why
the min_guarantee check is there specifically (or the other XML checks), and
nothing specific in the commit message for the min_guarantee block. Hence my
confusion and desire to document it, and hopefully save other people the
confusion, and prevent future XML validation checks being added there which
are safe to do at parse time.

So it seems like the XML validation checks are:

    if (qemuValidateCpuCount(vm->def, qemuCaps) < 0)
        return -1;

    if (!migration && !snapshot &&
        virDomainDefCheckDuplicateDiskInfo(vm->def) < 0)
        return -1;

    if (vm->def->mem.min_guarantee) {
        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                       _("Parameter 'min_guarantee' "
                         "not supported by QEMU."));
        return -1;
    }

And they are explicitly done at process startup time for back compat reasons.
So I'll stick them in a function like qemuProcessStartValidateXML(), with a
comment explaining why here and not at parse time. Sound good?

Thanks,
Cole