Re: [Qemu-devel] [RFC PATCH v2 14/23] cpus: Convert cpu_index into a bitmap

[David Gibson]

On Mon, Mar 23, 2015 at 07:05:55PM +0530, Bharata B Rao 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.
> 
> Signed-off-by: Bharata B Rao <address@hidden>
> ---
>  exec.c            | 37 ++++++++++++++++++++++++++++++++++---
>  include/qom/cpu.h |  8 ++++++++
>  2 files changed, 42 insertions(+), 3 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index e1ff6b0..9bbab02 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -527,21 +527,52 @@ 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 max_cpus;
> +    } else {
> +        bitmap_set(cpu_index_map, cpu, 1);
> +        return cpu;
> +    }
> +}
> +
> +void cpu_exec_exit(CPUState *cpu)
> +{
> +    bitmap_clear(cpu_index_map, cpu->cpu_index, 1);
> +}

AFAICT, this function is never called, which seems like a bug.

> +#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;
> -
>  #if defined(CONFIG_USER_ONLY)
> +    CPUState *some_cpu;
> +
>      cpu_list_lock();
> -#endif
>      cpu_index = 0;
>      CPU_FOREACH(some_cpu) {
>          cpu_index++;
>      }
>      cpu->cpu_index = cpu_index;
> +#else
> +    Error *local_err = NULL;
> +
> +    cpu_index = cpu->cpu_index = cpu_get_free_index(&local_err);
> +    if (local_err) {
> +        error_propagate(errp, local_err);
> +        return;
> +    }
> +#endif
>      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 48fd6fb..5241cf4 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -659,6 +659,14 @@ void cpu_watchpoint_remove_all(CPUState *cpu, int mask);
>  void QEMU_NORETURN cpu_abort(CPUState *cpu, const char *fmt, ...)
>      GCC_FMT_ATTR(2, 3);
>  
> +#ifndef CONFIG_USER_ONLY
> +void cpu_exec_exit(CPUState *cpu);
> +#else
> +static inline void cpu_exec_exit(CPUState *cpu)
> +{
> +}
> +#endif
> +
>  #ifdef CONFIG_SOFTMMU
>  extern const struct VMStateDescription vmstate_cpu_common;
>  #else

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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