Re: [Qemu-devel] [RFC v0 PATCH] cpus: Convert cpu_index into a bitmap

[Bharata B Rao]

On Tue, Mar 17, 2015 at 07:56:41AM +0100, Alexander Graf wrote:
> 
> 
> On 13.03.15 12:56, Bharata B Rao wrote:
> > From: Bharata B Rao <address@hidden>
> > 
> > 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.
> > 
> > I am not sure if this is the right and an acceptable approach. The
> > alternative is to do something similar for PowerPC alone and not
> > depend on cpu_index.
> > 
> > I have tested this with out-of-the-tree patches for CPU hot plug and
> > removal on x86 and sPAPR PowerPC.
> > 
> > Signed-off-by: Bharata B Rao <address@hidden>
> > ---
> >  exec.c                      | 39 +++++++++++++++++++++++++++++----------
> >  include/exec/exec-all.h     |  1 +
> >  target-alpha/cpu.c          |  6 ++++++
> >  target-arm/cpu.c            |  1 +
> >  target-cris/cpu.c           |  6 ++++++
> >  target-i386/cpu.c           |  6 ++++++
> >  target-lm32/cpu.c           |  6 ++++++
> >  target-m68k/cpu.c           |  6 ++++++
> >  target-microblaze/cpu.c     |  6 ++++++
> >  target-mips/cpu.c           |  6 ++++++
> >  target-moxie/cpu.c          |  6 ++++++
> >  target-openrisc/cpu.c       |  6 ++++++
> >  target-ppc/translate_init.c |  6 ++++++
> >  target-s390x/cpu.c          |  1 +
> >  target-sh4/cpu.c            |  6 ++++++
> >  target-sparc/cpu.c          |  1 +
> >  target-tricore/cpu.c        |  5 +++++
> >  target-unicore32/cpu.c      |  6 ++++++
> >  target-xtensa/cpu.c         |  6 ++++++
> >  19 files changed, 116 insertions(+), 10 deletions(-)
> > 
> > diff --git a/exec.c b/exec.c
> > index e97071a..7760f2d 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -530,21 +530,40 @@ void tcg_cpu_address_space_init(CPUState *cpu, 
> > AddressSpace *as)
> >  }
> >  #endif
> >  
> > +static DECLARE_BITMAP(cpu_index_map, MAX_CPUMASK_BITS);
> > +
> > +#ifdef CONFIG_USER_ONLY
> > +int max_cpus = 1; /* TODO: Check if this is correct ? */
> > +#endif
> > +
> > +static int cpu_get_free_index(void)
> > +{
> > +    int cpu = find_first_zero_bit(cpu_index_map, max_cpus);
> > +
> > +    if (cpu == max_cpus) {
> > +        fprintf(stderr, "WARNING: qemu: 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);
> > +}
> > +
> >  void cpu_exec_init(CPUArchState *env)
> >  {
> >      CPUState *cpu = ENV_GET_CPU(env);
> >      CPUClass *cc = CPU_GET_CLASS(cpu);
> > -    CPUState *some_cpu;
> > -    int cpu_index;
> >  
> >  #if defined(CONFIG_USER_ONLY)
> >      cpu_list_lock();
> >  #endif
> > -    cpu_index = 0;
> > -    CPU_FOREACH(some_cpu) {
> > -        cpu_index++;
> > -    }
> > -    cpu->cpu_index = cpu_index;
> > +    cpu->cpu_index = cpu_get_free_index();
> >      cpu->numa_node = 0;
> >      QTAILQ_INIT(&cpu->breakpoints);
> >      QTAILQ_INIT(&cpu->watchpoints);
> > @@ -558,16 +577,16 @@ void cpu_exec_init(CPUArchState *env)
> >      cpu_list_unlock();
> >  #endif
> >      if (qdev_get_vmsd(DEVICE(cpu)) == NULL) {
> > -        vmstate_register(NULL, cpu_index, &vmstate_cpu_common, cpu);
> > +        vmstate_register(NULL, cpu->cpu_index, &vmstate_cpu_common, cpu);
> >      }
> >  #if defined(CPU_SAVE_VERSION) && !defined(CONFIG_USER_ONLY)
> > -    register_savevm(NULL, "cpu", cpu_index, CPU_SAVE_VERSION,
> > +    register_savevm(NULL, "cpu", cpu->cpu_index, CPU_SAVE_VERSION,
> >                      cpu_save, cpu_load, env);
> >      assert(cc->vmsd == NULL);
> >      assert(qdev_get_vmsd(DEVICE(cpu)) == NULL);
> >  #endif
> >      if (cc->vmsd != NULL) {
> > -        vmstate_register(NULL, cpu_index, cc->vmsd, cpu);
> > +        vmstate_register(NULL, cpu->cpu_index, cc->vmsd, cpu);
> >      }
> >  }
> >  
> > diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
> > index 8eb0db3..95fbba0 100644
> > --- a/include/exec/exec-all.h
> > +++ b/include/exec/exec-all.h
> > @@ -89,6 +89,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
> >                                target_ulong pc, target_ulong cs_base, int 
> > flags,
> >                                int cflags);
> >  void cpu_exec_init(CPUArchState *env);
> > +void cpu_exec_exit(CPUState *cpu);
> >  void QEMU_NORETURN cpu_loop_exit(CPUState *cpu);
> >  int page_unprotect(target_ulong address, uintptr_t pc, void *puc);
> >  void tb_invalidate_phys_page_range(tb_page_addr_t start, tb_page_addr_t 
> > end,
> > diff --git a/target-alpha/cpu.c b/target-alpha/cpu.c
> > index a98b7d8..7c57165 100644
> > --- a/target-alpha/cpu.c
> > +++ b/target-alpha/cpu.c
> > @@ -250,6 +250,11 @@ static const TypeInfo ev68_cpu_type_info = {
> >      .parent = TYPE("ev67"),
> >  };
> >  
> > +static void alpha_cpu_finalize(Object *obj)
> > +{
> > +    cpu_exec_exit(CPU(obj));
> > +}
> > +
> >  static void alpha_cpu_initfn(Object *obj)
> >  {
> >      CPUState *cs = CPU(obj);
> > @@ -305,6 +310,7 @@ static const TypeInfo alpha_cpu_type_info = {
> >      .parent = TYPE_CPU,
> >      .instance_size = sizeof(AlphaCPU),
> >      .instance_init = alpha_cpu_initfn,
> > +    .instance_finalize = alpha_cpu_finalize,
> 
> Would it be possible to put this into TYPE_CPU->instance_finalize instead?

Yes possible and that would be much cleaner since I wouldn't have to touch
all archs. But it will be asymmetric in some sense as cpu_exec_init() is
called from all individual cpus' instance_init but cpu_exec_exit() will be
called from parent's (TYPE_CPU) instance_finalize. If that is fine, I shall
post v2 with this change.

Regards,
Bharata.