Re: [PATCH v2 1/2] xen/sched: core: skip missing vcpu slots in sched_move_domain()

[Jan Beulich <jbeulich@suse.com>]

On 19.08.2026 09:28, Furkan Çalışkan wrote:
> 
> 
> On 8/19/26 09:53, Jan Beulich wrote:
>> On 19.08.2026 07:15, Furkan Caliskan wrote:
>>> --- a/xen/common/sched/core.c
>>> +++ b/xen/common/sched/core.c
>>> @@ -745,6 +745,38 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
>>>  
>>>      for ( unit_idx = 0; unit_idx < n_units; unit_idx++ )
>>>      {
>>> +        /*
>>> +         * A vcpu slot can be missing if creation failed partway
>>> +         * through. A dying domain is being torn down regardless, so
>>> +         * skip the unit -- but a domain that isn't dying still needs
>>> +         * every vcpu it has schedulable, so fail instead of silently
>>> +         * dropping some of them.
>>> +         */
>>> +        bool vcpu_failed = false;
>>> +
>>> +        for ( unsigned int i = 0;
>>> +              i < gran && unit_idx * gran + i < d->max_vcpus; i++ )
>>> +        {
>>> +            if ( !d->vcpu[unit_idx * gran + i] )
>>
>> Is there a particular reason domain_vcpu() cannot be used here?
> 
> We still need to guard against d->max_vcpus so that out-of-bounds
> indices in a partially filled unit don't get treated as missing 
> vCPUs by domain_vcpu() returning NULL. 

Ah, right - the bounds check cannot really be folded here. Then ...

> However, domain_vcpu(unit_idx * gran + i) can be used here instead of
> d->vcpu[unit_idx * gran + i]. I simply used d->vcpu[] because the rest
> of the function uses it that way.

... best wait for Jürgen to comment. Outside of the scheduler we're
trying to replace open-coding of domain_vcpu(), but inside the
scheduler things may be different.

Jan