Re: [PATCH v7 05/15] machine: Improve the error reporting of smp parsing

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 8/23/21 2:27 PM, Yanan Wang wrote:
> We have two requirements for a valid SMP configuration:
> the product of "sockets * cores * threads" must represent all the
> possible cpus, i.e., max_cpus, and then must include the initially
> present cpus, i.e., smp_cpus.
> 
> So we only need to ensure 1) "sockets * cores * threads == maxcpus"
> at first and then ensure 2) "maxcpus >= cpus". With a reasonable
> order of the sanity check, we can simplify the error reporting code.
> When reporting an error message we also report the exact value of
> each topology member to make users easily see what's going on.
> 
> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
> Reviewed-by: Andrew Jones <drjones@redhat.com>
> Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
> ---
>  hw/core/machine.c | 22 +++++++++-------------
>  hw/i386/pc.c      | 24 ++++++++++--------------
>  2 files changed, 19 insertions(+), 27 deletions(-)
> 
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 85908abc77..093c0d382d 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -779,25 +779,21 @@ static void smp_parse(MachineState *ms, SMPConfiguration *config, Error **errp)
>      maxcpus = maxcpus > 0 ? maxcpus : sockets * cores * threads;
>      cpus = cpus > 0 ? cpus : maxcpus;
>  
> -    if (sockets * cores * threads < cpus) {
> -        error_setg(errp, "cpu topology: "
> -                   "sockets (%u) * cores (%u) * threads (%u) < "
> -                   "smp_cpus (%u)",
> -                   sockets, cores, threads, cpus);
> +    if (sockets * cores * threads != maxcpus) {
> +        error_setg(errp, "Invalid CPU topology: "
> +                   "product of the hierarchy must match maxcpus: "
> +                   "sockets (%u) * cores (%u) * threads (%u) "
> +                   "!= maxcpus (%u)",
> +                   sockets, cores, threads, maxcpus);
>          return;
>      }

Thinking about scalability, MachineClass could have a
parse_cpu_topology() handler, and this would be the
generic one. Principally because architectures don't
use the same terms, and die/socket/core/thread arrangement
is machine specific (besides being arch-spec).
Not a problem as of today, but the way we try to handle
this generically seems over-engineered to me.

[unrelated to this particular patch]