Re: [Qemu-devel] [Qemu-ppc] [PATCH] spapr: check smp_threads < vsmt

[Greg Kurz <groug@kaod.org>]

On Wed,  7 Feb 2018 17:17:30 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> We ignore silently the value of smp_threads when we set
> the VSMT value, and if smp_threads is greater than VSMT
> kernel is going into trouble later.
> 
> Fixes: 8904e5a750
> ("spapr: Adjust default VSMT value for better migration compatibility")
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  hw/ppc/spapr.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 32a876be56..638b3cafd0 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2294,15 +2294,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
>      }
>  
>      /* Detemine the VSMT mode to use: */
> -    if (vsmt_user) {
> -        if (spapr->vsmt < smp_threads) {
> -            error_setg(&local_err, "Cannot support VSMT mode %d"
> -                         " because it must be >= threads/core (%d)",
> -                         spapr->vsmt, smp_threads);

In this case, we'd error out because the user requested two settings that
are known to be incompatible.

> -            goto out;
> -        }
> -        /* In this case, spapr->vsmt has been set by the command line */
> -    } else {
> +    if (!vsmt_user) {
>          /*
>           * Default VSMT value is tricky, because we need it to be as
>           * consistent as possible (for migration), but this requires
> @@ -2313,6 +2305,13 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
>          spapr->vsmt = 8;

In this case, we'd error out because the user requested a setting that is
incompatible with our default. Maybe worth a separate error message ?

"Cannot support %d threads/core because it must be <= to default VSMT mode (8)"

Anyway, the fix is good:

Reviewed-by: Greg Kurz <groug@kaod.org>

>      }
>  
> +    if (spapr->vsmt < smp_threads) {
> +        error_setg(&local_err, "Cannot support VSMT mode %d"
> +                   " because it must be >= threads/core (%d)",
> +                   spapr->vsmt, smp_threads);
> +        goto out;
> +    }
> +
>      /* KVM: If necessary, set the SMT mode: */
>      if (kvm_enabled() && (spapr->vsmt != kvm_smt)) {
>          ret = kvmppc_set_smt_threads(spapr->vsmt);