Re: [PATCH v2 5/7] spapr: TCG allow up to 8-thread SMT on POWER8 and newer CPUs

[Cédric Le Goater]

On 6/22/23 11:33, Nicholas Piggin wrote:
> PPC TCG supports SMT CPU configurations for non-hypervisor state, so
> permit POWER8-10 pseries machines to enable SMT.
>
> This requires PIR and TIR be set, because that's how sibling thread
> matching is done by TCG.
>
> spapr's nested-HV capability does not currently coexist with SMT, so
> that combination is prohibited (interestingly somewhat analogous to
> LPAR-per-core mode on real hardware which also does not support KVM).
>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   hw/ppc/spapr.c          | 16 ++++++++++++----
>   hw/ppc/spapr_caps.c     | 14 ++++++++++++++
>   hw/ppc/spapr_cpu_core.c |  7 +++++--
>   3 files changed, 31 insertions(+), 6 deletions(-)
>
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 8e7d497f25..677b5eef9d 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2525,10 +2525,18 @@ static void spapr_set_vsmt_mode(SpaprMachineState 
> *spapr, Error **errp)
>       int ret;
>       unsigned int smp_threads = ms->smp.threads;
>   
> -    if (tcg_enabled() && (smp_threads > 1)) {
> -        error_setg(errp, "TCG cannot support more than 1 thread/core "
> -                   "on a pseries machine");
> -        return;
> +    if (tcg_enabled()) {
> +        if (!ppc_type_check_compat(ms->cpu_type, CPU_POWERPC_LOGICAL_2_07, 0,
> +                                   spapr->max_compat_pvr)) {
> +            error_setg(errp, "TCG only supports SMT on POWER8 or newer CPUs");
> +            return;
> +        }

So if TCG is enabled and the CPU < P8, QEMU bails out ? You should run
qemu-ppc-boot :)

Thanks,

C.

> +
> +        if (smp_threads > 8) {
> +            error_setg(errp, "TCG cannot support more than 8 threads/core "
> +                       "on a pseries machine");
> +            return;
> +        }
>       }
>       if (!is_power_of_2(smp_threads)) {
>           error_setg(errp, "Cannot support %d threads/core on a pseries "
> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> index 3fd45a6dec..5a0755d34f 100644
> --- a/hw/ppc/spapr_caps.c
> +++ b/hw/ppc/spapr_caps.c
> @@ -473,6 +473,20 @@ static void cap_nested_kvm_hv_apply(SpaprMachineState 
> *spapr,
>                   error_append_hint(errp,
>                                     "Try appending -machine 
> cap-nested-hv=off\n");
>           }
> +    } else if (tcg_enabled()) {
> +        MachineState *ms = MACHINE(spapr);
> +        unsigned int smp_threads = ms->smp.threads;
> +
> +        /*
> +         * Nested-HV vCPU env state to L2, so SMT-shared SPR updates, for
> +         * example, do not necessarily update the correct SPR value on sibling
> +         * threads that are in a different guest/host context.
> +         */
> +        if (smp_threads > 1) {
> +            error_setg(errp, "TCG does not support nested-HV with SMT");
> +            error_append_hint(errp, "Try appending -machine cap-nested-hv=off "
> +                                    "or use threads=1 with -smp\n");
> +        }
>       }
>   }
>   
> diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> index 9b88dd549a..a4e3c2fadd 100644
> --- a/hw/ppc/spapr_cpu_core.c
> +++ b/hw/ppc/spapr_cpu_core.c
> @@ -255,7 +255,7 @@ static void spapr_cpu_core_unrealize(DeviceState *dev)
>   }
>   
>   static bool spapr_realize_vcpu(PowerPCCPU *cpu, SpaprMachineState *spapr,
> -                               SpaprCpuCore *sc, Error **errp)
> +                               SpaprCpuCore *sc, int thread_index, Error 
> **errp)
>   {
>       CPUPPCState *env = &cpu->env;
>       CPUState *cs = CPU(cpu);
> @@ -267,6 +267,9 @@ static bool spapr_realize_vcpu(PowerPCCPU *cpu, 
> SpaprMachineState *spapr,
>       cpu_ppc_set_vhyp(cpu, PPC_VIRTUAL_HYPERVISOR(spapr));
>       kvmppc_set_papr(cpu);
>   
> +    env->spr_cb[SPR_PIR].default_value = cs->cpu_index;
> +    env->spr_cb[SPR_TIR].default_value = thread_index;
> +
>       /* Set time-base frequency to 512 MHz. vhyp must be set first. */
>       cpu_ppc_tb_init(env, SPAPR_TIMEBASE_FREQ);
>   
> @@ -337,7 +340,7 @@ static void spapr_cpu_core_realize(DeviceState *dev, Error **errp)
>       for (i = 0; i < cc->nr_threads; i++) {
>           sc->threads[i] = spapr_create_vcpu(sc, i, errp);
>           if (!sc->threads[i] ||
> -            !spapr_realize_vcpu(sc->threads[i], spapr, sc, errp)) {
> +            !spapr_realize_vcpu(sc->threads[i], spapr, sc, i, errp)) {
>               spapr_cpu_core_unrealize(dev);
>               return;
>           }