Hi Prasad,
On Mon, 4 Mar 2024 at 12:19, Zhao Liu <zhao1.liu@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