Re: [Qemu-ppc] [PATCH v5 3/4] spapr: Implement H_CONFER

[Greg Kurz]

On Wed, 17 Jul 2019 15:39:51 +1000
Nicholas Piggin <address@hidden> wrote:

> This does not do directed yielding and is not quite as strict as PAPR
> specifies in terms of precise dispatch behaviour. This generally will
> mean suboptimal performance, rather than guest misbehaviour. Linux
> does not rely on exact dispatch behaviour.
> 
> Signed-off-by: Nicholas Piggin <address@hidden>
> ---

LGTM.

Reviewed-by: Greg Kurz <address@hidden>

Just two minor comments, see below.

> Changes since v4:
> - Style, added justification comments, spelling.
> - Fixed trying to dereference spapr_cpu for a -1 target.
> 
>  hw/ppc/spapr_hcall.c | 68 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
> 
> diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> index 8b208ab259..5e655172b2 100644
> --- a/hw/ppc/spapr_hcall.c
> +++ b/hw/ppc/spapr_hcall.c
> @@ -1069,6 +1069,73 @@ static target_ulong h_cede(PowerPCCPU *cpu, 
> SpaprMachineState *spapr,
>      return H_SUCCESS;
>  }
>  
> +static target_ulong h_confer(PowerPCCPU *cpu, SpaprMachineState *spapr,
> +                           target_ulong opcode, target_ulong *args)
> +{
> +    target_long target = args[0];
> +    uint32_t dispatch = args[1];
> +    CPUState *cs = CPU(cpu);
> +    SpaprCpuState *spapr_cpu;
> +
> +    /*
> +     * -1 means confer to all other CPUs without dispatch counter check,
> +     *  otherwise it's a targeted confer.
> +     */
> +    if (target != -1) {
> +        PowerPCCPU *target_cpu = spapr_find_cpu(target);
> +        CPUState *target_cs = CPU(target_cpu);
> +        unsigned int target_dispatch;

Maybe make it uint32_t to be consistent with dispatch above, and this
is the actual return type of ldl_be_phys() ?

> +
> +        if (!target_cs) {

This is the only user of target_cs, maybe drop it and use target_cpu
instead ?

> +            return H_PARAMETER;
> +        }
> +
> +        spapr_cpu = spapr_cpu_state(target_cpu);
> +
> +        /*
> +         * target == self is a special case, we wait until prodded, without
> +         * dispatch counter check.
> +         */
> +        if (cpu == target_cpu) {
> +            if (spapr_cpu->prod) {
> +                spapr_cpu->prod = false;
> +
> +                return H_SUCCESS;
> +            }
> +
> +            cs->halted = 1;
> +            cs->exception_index = EXCP_HALTED;
> +            cs->exit_request = 1;
> +
> +            return H_SUCCESS;
> +        }
> +
> +        if (!spapr_cpu->vpa_addr || ((dispatch & 1) == 0)) {
> +            return H_SUCCESS;
> +        }
> +
> +        target_dispatch = ldl_be_phys(cs->as,
> +                                  spapr_cpu->vpa_addr + 
> VPA_DISPATCH_COUNTER);
> +        if (target_dispatch != dispatch) {
> +            return H_SUCCESS;
> +        }
> +
> +        /*
> +         * The targeted confer does not do anything special beyond yielding
> +         * the current vCPU, but even this should be better than nothing.
> +         * At least for single-threaded tcg, it gives the target a chance to
> +         * run before we run again. Multi-threaded tcg does not really do
> +         * anything with EXCP_YIELD yet.
> +         */
> +    }
> +
> +    cs->exception_index = EXCP_YIELD;
> +    cs->exit_request = 1;
> +    cpu_loop_exit(cs);
> +
> +    return H_SUCCESS;
> +}
> +
>  static target_ulong h_prod(PowerPCCPU *cpu, SpaprMachineState *spapr,
>                             target_ulong opcode, target_ulong *args)
>  {
> @@ -1909,6 +1976,7 @@ static void hypercall_register_types(void)
>      /* hcall-splpar */
>      spapr_register_hypercall(H_REGISTER_VPA, h_register_vpa);
>      spapr_register_hypercall(H_CEDE, h_cede);
> +    spapr_register_hypercall(H_CONFER, h_confer);
>      spapr_register_hypercall(H_PROD, h_prod);
>  
>      spapr_register_hypercall(H_SIGNAL_SYS_RESET, h_signal_sys_reset);