Re: [PATCH] spapr: Fix VSMT mode when it is not supported by the kernel

[Greg Kurz <groug@kaod.org>]

On Fri,  8 Nov 2019 16:40:35 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> Commit 29cb4187497d sets by default the VSMT to smp_threads,
> but older kernels (< 4.13) don't support that.
> 
> We can reasonably restore previous behavior with this kernel
> to allow to run QEMU as before.
> 
> If VSMT is not supported, VSMT will be set to MAX(8, smp_threads)
> as it is done for previous machine types (< pseries-4.2)
> 

It is usually _bad_ to base the machine behavior on host capabilities.
What happens if we migrate between an older kernel and a recent one ?

I understand this is to fix tests/migration-test on older kernels.
Couldn't this be achieved with migration-test doing some introspection
and maybe pass vsmt=8 on the QEMU command line ?

> Fixes: 29cb4187497d ("spapr: Set VSMT to smp_threads by default")
> Cc: groug@kaod.org
> Reported-by: Lukáš Doktor <ldoktor@redhat.com>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  hw/ppc/spapr.c       | 2 +-
>  target/ppc/kvm.c     | 5 +++++
>  target/ppc/kvm_ppc.h | 6 ++++++
>  3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 94f9d27096af..f6c8ad1eda32 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2522,7 +2522,7 @@ static void spapr_set_vsmt_mode(SpaprMachineState *spapr, Error **errp)
>              goto out;
>          }
>          /* In this case, spapr->vsmt has been set by the command line */
> -    } else if (!smc->smp_threads_vsmt) {
> +    } else if (!smc->smp_threads_vsmt || !kvmppc_check_smt_possible()) {
>          /*
>           * Default VSMT value is tricky, because we need it to be as
>           * consistent as possible (for migration), but this requires
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 7d2e8969ac5f..40ed59881167 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2060,6 +2060,11 @@ void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy)
>      }
>  }
>  
> +bool kvmppc_check_smt_possible(void)
> +{
> +    return kvm_enabled() && cap_ppc_smt_possible;
> +}
> +
>  int kvmppc_smt_threads(void)
>  {
>      return cap_ppc_smt ? cap_ppc_smt : 1;
> diff --git a/target/ppc/kvm_ppc.h b/target/ppc/kvm_ppc.h
> index 98bd7d5da6d6..c9629a416b0b 100644
> --- a/target/ppc/kvm_ppc.h
> +++ b/target/ppc/kvm_ppc.h
> @@ -27,6 +27,7 @@ void kvmppc_enable_h_page_init(void);
>  void kvmppc_set_papr(PowerPCCPU *cpu);
>  int kvmppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr);
>  void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy);
> +bool kvmppc_check_smt_possible(void);
>  int kvmppc_smt_threads(void);
>  void kvmppc_hint_smt_possible(Error **errp);
>  int kvmppc_set_smt_threads(int smt);
> @@ -159,6 +160,11 @@ static inline void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy)
>  {
>  }
>  
> +static inline bool kvmppc_check_smt_possible(void)
> +{
> +    return false;
> +}
> +
>  static inline int kvmppc_smt_threads(void)
>  {
>      return 1;