Re: [PATCH for-6.1 v2] machine: Disallow specifying topology parameters as zero

["wangyanan (Y)" <wangyanan55@huawei.com>]

On 2021/7/23 16:02, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
>
>> On Thu, Jul 22, 2021 at 11:43:26PM +0800, Yanan Wang wrote:
>>> In the SMP configuration, we should either specify a topology
>>> parameter with a reasonable value (equal to or greater than 1)
>>> or just leave it omitted and QEMU will calculate its value.
>>> Configurations which explicitly specify the topology parameters
>>> as zero like "sockets=0" are meaningless, so disallow them.
>>>
>>> However, the commit 1e63fe685804d
>>> (machine: pass QAPI struct to mc->smp_parse) has documented that
>>> '0' has the same semantics as omitting a parameter in the qapi
>>> comment for SMPConfiguration. So this patch fixes the doc and
>>> also adds the corresponding sanity check in the smp parsers.
>>>
>>> Suggested-by: Andrew Jones <drjones@redhat.com>
>>> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
>>> ---
>>>   hw/core/machine.c | 14 ++++++++++++++
>>>   qapi/machine.json |  6 +++---
>>>   qemu-options.hx   | 12 +++++++-----
>>>   3 files changed, 24 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/hw/core/machine.c b/hw/core/machine.c
>>> index 775add0795..db129d937b 100644
>>> --- a/hw/core/machine.c
>>> +++ b/hw/core/machine.c
>>> @@ -829,6 +829,20 @@ static void machine_set_smp(Object *obj, Visitor *v, const char *name,
>>>           return;
>>>       }
>>>   
>>> +    /*
>>> +     * The topology parameters must be specified equal to or great than one
>>> +     * or just omitted, explicit configuration like "cpus=0" is not allowed.
>>> +     */
>>> +    if ((config->has_cpus && config->cpus == 0) ||
>>> +        (config->has_sockets && config->sockets == 0) ||
>>> +        (config->has_dies && config->dies == 0) ||
>>> +        (config->has_cores && config->cores == 0) ||
>>> +        (config->has_threads && config->threads == 0) ||
>>> +        (config->has_maxcpus && config->maxcpus == 0)) {
>>> +        error_setg(errp, "parameters must be equal to or greater than one if provided");
>> I'd suggest a slight tweak since when seen it lacks context:
>>
>> $ ./qemu-system-x86_64 -smp 4,cores=0,sockets=2
>> qemu-system-x86_64: parameters must be equal to or greater than one if provided
>>
>>
>>      error_setg(errp, "CPU topology parameters must be equal to or greater than one if provided");
> Let's scratch "if provided".
>
> I'd replace "must be equal to or greater than one" by "must be
> positive", or maybe "must be greater than zero".
How about we use "must be greater than zero" ?
After a grep search of these two sentences in QEMU, they both show up
in several places. "must be positive" always reports an invalid value that
is "< 0". While the check in this patch actually reject an invalid zero 
value.
>>> diff --git a/qemu-options.hx b/qemu-options.hx
>>> index 99ed5ec5f1..b0168f8c48 100644
>>> --- a/qemu-options.hx
>>> +++ b/qemu-options.hx
>>> @@ -223,11 +223,13 @@ SRST
>>>       of computing the CPU maximum count.
>>>   
>>>       Either the initial CPU count, or at least one of the topology parameters
>>> -    must be specified. Values for any omitted parameters will be computed
>>> -    from those which are given. Historically preference was given to the
>>> -    coarsest topology parameters when computing missing values (ie sockets
>>> -    preferred over cores, which were preferred over threads), however, this
>>> -    behaviour is considered liable to change.
>>> +    must be specified. The specified parameters must be equal to or great
>> s/great/greater/
>>
>>> +    than one, explicit configuration like "cpus=0" is not allowed. Values
> "positive" again.
Thanks,
Yanan
>>> +    for any omitted parameters will be computed from those which are given.
>>> +    Historically preference was given to the coarsest topology parameters
>>> +    when computing missing values (ie sockets preferred over cores, which
>>> +    were preferred over threads), however, this behaviour is considered
>>> +    liable to change.
>>>   ERST
>>
>> If you make the text changes, then feel free to add this when posting v2:
>>
>>   Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
>>   Tested-by: Daniel P. Berrangé <berrange@redhat.com>
>>   
>>
>>
>> Regards,
>> Daniel
> .