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

[Markus Armbruster <armbru@redhat.com>]

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".

>> 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.

>> +    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