Re: [PATCH] hw/core/machine-smp: Remove deprecated "parameter=0" SMP configurations
Hi Prasad,
On Mon, 4 Mar 2024 at 12:19, Zhao Liu
<zhao1.liu(a)linux.intel.com> wrote:
> > unsigned maxcpus = config->has_maxcpus ? config->maxcpus : 0;
>
> This indicates the default maxcpus is initialized as 0 if user doesn't
> specifies it.
* 'has_maxcpus' should be set only if maxcpus > 0. If maxcpus == 0,
then setting 'has_maxcpus=1' seems convoluted.
After simple test, if user sets maxcpus as 0, the has_maxcpus will be
true as well...I think it's related with QAPI code generation logic.
> However, we could initialize maxcpus as other default value,
e.g.,
>
> maxcpus = config->has_maxcpus ? config->maxcpus : 1.
===
hw/core/machine.c
machine_initfn
/* default to mc->default_cpus */
ms->smp.cpus = mc->default_cpus;
ms->smp.max_cpus = mc->default_cpus;
static void machine_class_base_init(ObjectClass *oc, void *data)
{
MachineClass *mc = MACHINE_CLASS(oc);
mc->max_cpus = mc->max_cpus ?: 1;
mc->min_cpus = mc->min_cpus ?: 1;
mc->default_cpus = mc->default_cpus ?: 1;
}
===
* Looking at the above bits, it seems smp.cpus & smp.max_cpus are
initialised to 1 via default_cpus in MachineClass object.
Yes.
The maxcpus I mentioned is a local virable in
machine_parse_smp_config(), whihc is used to do sanity-check check.
In machine_parse_smp_config(), when we can confirm the topology is
valid, then ms->smp.cpus and ms->smp.max_cpus are set with the valid
virables (cpus and maxcpus).
>> if (config->has_maxcpus && config->maxcpus ==
0)
> This check only wants to identify the case that user sets the 0.
> If the default maxcpus is initialized as 0, then (maxcpus == 0) will
> fail if user doesn't set maxcpus.
>
> But it is still necessary to distinguish whether maxcpus is user-set or
> auto-initialized.
* If it is set to zero(0) either by user or by auto-initialise, it is
still invalid, right?
The latter, "auto-initialise", means user could omit "cpus" and
"maxcpus"
parameters in -smp.
Even though the local variable "cpus" and "maxcpus" are initialized
as
0, eventually ms->smp.cpus and ms->smp.max_cpus will still have the
valid values.
> If it is user-set, -smp should fail is there's invalid
maxcpus/invalid
> topology.
>
> Otherwise, if it is auto-initialized, its value should be adjusted based
> on other topology components as the above calculation in (*).
* Why have such diverging ways?
* Could we simplify it as
- If cpus/maxcpus==0, it is invalid, show an error and exit.
Hmm, the origial behavior means if user doesn't set cpus=*/maxcpus=* in
-smp, then QEMU will auto-complete these 2 fields.
If we also return error for the above case that user omits cpus and
maxcpus parameters, then this change the QEMU's API and we need to mark
feature that the cpus/maxcpus parameter can be omitted as deprecated and
remove it out. Just like what I did in this patch for zeroed-parameter
case.
I feel if there's no issue then it's not necessary to change the API. Do
you agree?
- If cpus/maxcpus > 0, but incorrect for topology, then
re-calculate the correct value based on topology parameters. If the
re-calculated value is still incorrect or unsatisfactory, then show an
error and exit.
Yes, this case is right.
* Saying that user setting cpu/maxcpus=0 is invalid and
auto-initialising it to zero(0) is valid, is not consistent.
I think "auto-initialising it to zero(0)" doesn't means we re-initialize
ms->smp.cpus and ms->smp.max_cpus as 0 (these 2 fields store actual basic
topology information and they're defult as 1 as you said above).
Does my explaination address your concern? ;-)
Thanks,
Zhao