Re: [PULL 06/28] spapr: Set VSMT to smp_threads by default

[Laurent Vivier]

On 24/10/2019 10:17, David Gibson wrote:
> From: Greg Kurz <address@hidden>
> 
> Support for setting VSMT is available in KVM since linux-4.13. Most distros
> that support KVM on POWER already have it. It thus seem reasonable enough
> to have the default machine to set VSMT to smp_threads.
> 
> This brings contiguous VCPU ids and thus brings their upper bound down to
> the machine's max_cpus. This is especially useful for XIVE KVM devices,
> which may thus allocate only one VP descriptor per VCPU.
> 
> Signed-off-by: Greg Kurz <address@hidden>
> Message-Id: <address@hidden>
> Signed-off-by: David Gibson <address@hidden>
> ---
>  hw/ppc/spapr.c         | 7 ++++++-
>  include/hw/ppc/spapr.h | 1 +
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 4eb97d3a9b..428b834f30 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2496,6 +2496,7 @@ static CPUArchId *spapr_find_cpu_slot(MachineState *ms, 
> uint32_t id, int *idx)
>  static void spapr_set_vsmt_mode(SpaprMachineState *spapr, Error **errp)
>  {
>      MachineState *ms = MACHINE(spapr);
> +    SpaprMachineClass *smc = SPAPR_MACHINE_GET_CLASS(spapr);
>      Error *local_err = NULL;
>      bool vsmt_user = !!spapr->vsmt;
>      int kvm_smt = kvmppc_smt_threads();
> @@ -2522,7 +2523,7 @@ 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 {
> +    } else if (!smc->smp_threads_vsmt) {
>          /*
>           * Default VSMT value is tricky, because we need it to be as
>           * consistent as possible (for migration), but this requires
> @@ -2531,6 +2532,8 @@ static void spapr_set_vsmt_mode(SpaprMachineState 
> *spapr, Error **errp)
>           * overwhelmingly common case in production systems.
>           */
>          spapr->vsmt = MAX(8, smp_threads);
> +    } else {
> +        spapr->vsmt = smp_threads;
>      }
>  
>      /* KVM: If necessary, set the SMT mode: */
> @@ -4438,6 +4441,7 @@ static void spapr_machine_class_init(ObjectClass *oc, 
> void *data)
>      smc->irq = &spapr_irq_dual;
>      smc->dr_phb_enabled = true;
>      smc->linux_pci_probe = true;
> +    smc->smp_threads_vsmt = true;
>  }
>  
>  static const TypeInfo spapr_machine_info = {
> @@ -4505,6 +4509,7 @@ static void 
> spapr_machine_4_1_class_options(MachineClass *mc)
>  
>      spapr_machine_4_2_class_options(mc);
>      smc->linux_pci_probe = false;
> +    smc->smp_threads_vsmt = false;
>      compat_props_add(mc->compat_props, hw_compat_4_1, hw_compat_4_1_len);
>      compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
>  }
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index cbd1a4c9f3..2009eb64f9 100644
> --- a/include/hw/ppc/spapr.h
> +++ b/include/hw/ppc/spapr.h
> @@ -122,6 +122,7 @@ struct SpaprMachineClass {
>      bool broken_host_serial_model; /* present real host info to the guest */
>      bool pre_4_1_migration; /* don't migrate hpt-max-page-size */
>      bool linux_pci_probe;
> +    bool smp_threads_vsmt; /* set VSMT to smp_threads by default */
>  
>      void (*phb_placement)(SpaprMachineState *spapr, uint32_t index,
>                            uint64_t *buid, hwaddr *pio, 
> 

This patch breaks tests/migration-test on P8 host with kernel older than
4.3 because it tries by default to set the VSMT to 1.

qemu-system-ppc64: Failed to set KVM's VSMT mode to 1 (errno -22)
On PPC, a VM with 1 threads/core on a host with 8 threads/core requires
the use of VSMT mode 1.
This KVM seems to be too old to support VSMT.

As this is clearly intentional, is there a way to fix migration-test?

Thanks,
Laurent