Re: [PATCH for-next 3/6] xen/sched: Fix build when NR_CPUS == 1

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.02.2021 16:24, Connor Davis wrote:
> Return from cpu_schedule_up when either cpu is 0 or
> NR_CPUS == 1. This fixes the following:
> 
> core.c: In function 'cpu_schedule_up':
> core.c:2769:19: error: array subscript 1 is above array bounds
> of 'struct vcpu *[1]' [-Werror=array-bounds]
>  2769 |     if ( idle_vcpu[cpu] == NULL )
>       |
> 
> Signed-off-by: Connor Davis <connojdavis@xxxxxxxxx>
> ---
>  xen/common/sched/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
> index 9745a77eee..f5ec65bf9b 100644
> --- a/xen/common/sched/core.c
> +++ b/xen/common/sched/core.c
> @@ -2763,7 +2763,7 @@ static int cpu_schedule_up(unsigned int cpu)
>      cpumask_set_cpu(cpu, &sched_res_mask);
>  
>      /* Boot CPU is dealt with later in scheduler_init(). */
> -    if ( cpu == 0 )
> +    if ( cpu == 0 || NR_CPUS == 1 )
>          return 0;
>  
>      if ( idle_vcpu[cpu] == NULL )

I'm not convinced a compiler warning is due here, and in turn
I'm not sure we want/need to work around this the way you do.
First question is whether that's just a specific compiler
version that's flawed. If it's not just a special case (e.g.
some unreleased version) we may want to think of possible
alternatives - the addition looks really odd to me.

Jan