Re: [Qemu-devel] [PATCH v3 2/3] cpus: Convert cpu_index into a bitmap

[Peter Crosthwaite]

On Wed, May 20, 2015 at 10:02 PM, Bharata B Rao
<address@hidden> wrote:
> Currently CPUState.cpu_index is monotonically increasing and a newly
> created CPU always gets the next higher index. The next available
> index is calculated by counting the existing number of CPUs. This is
> fine as long as we only add CPUs, but there are architectures which
> are starting to support CPU removal too. For an architecture like PowerPC
> which derives its CPU identifier (device tree ID) from cpu_index, the
> existing logic of generating cpu_index values causes problems.
>
> With the currently proposed method of handling vCPU removal by parking
> the vCPU fd in QEMU
> (Ref: http://lists.gnu.org/archive/html/qemu-devel/2015-02/msg02604.html),
> generating cpu_index this way will not work for PowerPC.
>
> This patch changes the way cpu_index is handed out by maintaining
> a bit map of the CPUs that tracks both addition and removal of CPUs.
>
> The CPU bitmap allocation logic is part of cpu_exec_init() which is
> called by instance_init routines of various CPU targets. Newly added
> cpu_exec_exit() API handles the deallocation part and this routine is
> called from generic CPU::instance_finalize().
>
> Note: This new CPU enumeration is for !CONFIG_USER_ONLY only.
> CONFIG_USER_ONLY continues to have the old enumeration logic.
>
> Signed-off-by: Bharata B Rao <address@hidden>
> Reviewed-by: Eduardo Habkost <address@hidden>

Reviewed-by: Peter Crosthwaite <address@hidden>

> ---
>  exec.c            | 55 
> ++++++++++++++++++++++++++++++++++++++++++++++++++-----
>  include/qom/cpu.h |  1 +
>  qom/cpu.c         |  7 +++++++
>  3 files changed, 58 insertions(+), 5 deletions(-)
>
> diff --git a/exec.c b/exec.c
> index 5cf821e..dd688b8 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -518,21 +518,66 @@ void tcg_cpu_address_space_init(CPUState *cpu, 
> AddressSpace *as)
>  }
>  #endif
>
> +#ifndef CONFIG_USER_ONLY
> +static DECLARE_BITMAP(cpu_index_map, MAX_CPUMASK_BITS);
> +
> +static int cpu_get_free_index(Error **errp)
> +{
> +    int cpu = find_first_zero_bit(cpu_index_map, max_cpus);
> +
> +    if (cpu >= max_cpus) {
> +        error_setg(errp, "Trying to use more CPUs than allowed max of %d\n",
> +                    max_cpus);
> +        return -1;
> +    }
> +
> +    bitmap_set(cpu_index_map, cpu, 1);
> +    return cpu;
> +}
> +
> +void cpu_exec_exit(CPUState *cpu)
> +{
> +    if (cpu->cpu_index == -1) {
> +        /* cpu_index was never allocated by this @cpu or was already freed. 
> */
> +        return;
> +    }
> +
> +    bitmap_clear(cpu_index_map, cpu->cpu_index, 1);
> +    cpu->cpu_index = -1;
> +}
> +#else
> +
> +static int cpu_get_free_index(Error **errp)
> +{
> +    CPUState *some_cpu;
> +    int cpu_index = 0;
> +
> +    CPU_FOREACH(some_cpu) {
> +        cpu_index++;
> +    }
> +    return cpu_index;
> +}
> +
> +void cpu_exec_exit(CPUState *cpu)
> +{
> +}
> +#endif
> +
>  void cpu_exec_init(CPUArchState *env, Error **errp)
>  {
>      CPUState *cpu = ENV_GET_CPU(env);
>      CPUClass *cc = CPU_GET_CLASS(cpu);
> -    CPUState *some_cpu;
>      int cpu_index;
> +    Error *local_err = NULL;
>
>  #if defined(CONFIG_USER_ONLY)
>      cpu_list_lock();
>  #endif
> -    cpu_index = 0;
> -    CPU_FOREACH(some_cpu) {
> -        cpu_index++;
> +    cpu_index = cpu->cpu_index = cpu_get_free_index(&local_err);
> +    if (local_err) {
> +        error_propagate(errp, local_err);
> +        return;
>      }
> -    cpu->cpu_index = cpu_index;
>      cpu->numa_node = 0;
>      QTAILQ_INIT(&cpu->breakpoints);
>      QTAILQ_INIT(&cpu->watchpoints);
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index 39f0f19..7db310e 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -672,6 +672,7 @@ void cpu_watchpoint_remove_all(CPUState *cpu, int mask);
>
>  void QEMU_NORETURN cpu_abort(CPUState *cpu, const char *fmt, ...)
>      GCC_FMT_ATTR(2, 3);
> +void cpu_exec_exit(CPUState *cpu);
>
>  #ifdef CONFIG_SOFTMMU
>  extern const struct VMStateDescription vmstate_cpu_common;
> diff --git a/qom/cpu.c b/qom/cpu.c
> index 108bfa2..061a0c3 100644
> --- a/qom/cpu.c
> +++ b/qom/cpu.c
> @@ -312,9 +312,15 @@ static void cpu_common_initfn(Object *obj)
>      CPUState *cpu = CPU(obj);
>      CPUClass *cc = CPU_GET_CLASS(obj);
>
> +    cpu->cpu_index = -1;
>      cpu->gdb_num_regs = cpu->gdb_num_g_regs = cc->gdb_num_core_regs;
>  }
>
> +static void cpu_common_finalize(Object *obj)
> +{
> +    cpu_exec_exit(CPU(obj));
> +}
> +
>  static int64_t cpu_common_get_arch_id(CPUState *cpu)
>  {
>      return cpu->cpu_index;
> @@ -356,6 +362,7 @@ static const TypeInfo cpu_type_info = {
>      .parent = TYPE_DEVICE,
>      .instance_size = sizeof(CPUState),
>      .instance_init = cpu_common_initfn,
> +    .instance_finalize = cpu_common_finalize,
>      .abstract = true,
>      .class_size = sizeof(CPUClass),
>      .class_init = cpu_class_init,
> --
> 2.1.0
>
>