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

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Tue, Mar 17, 2015 at 11:56:04AM +0100, Andreas Färber wrote:
> Am 17.03.2015 um 09:39 schrieb 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 <bharata.rao@gmail.com>
> >>>
> >>> 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 <bharata@linux.vnet.ibm.com>
> >>> ---
> >>>  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.
> 
> Could you check: Wasn't there a patch from Fujitsu to move
> cpu_exec_init() to generic code? If both were generic, that would be
> fine. If that is problematic, I would accept the mismatch as long as it
> is "safe". That is, instance_finalize needs to handle any state of the
> object,

There is a patch from Zhu to move only vmstate_register related bits
from cpu_exec_init to cpu_common_realizefn.

http://lists.gnu.org/archive/html/qemu-devel/2015-01/msg01550.html

> and I think these two are better suited for realize/unrealize
> than instance_init/instance_finalize.

And Eduardo has a patch to move cpu_exec_init call from instance_init
to realize for target-i386.

https://lists.gnu.org/archive/html/qemu-devel/2015-03/msg01056.html

So if the preferred way is to call cpu_exec_init from realize, then
Eduardo - Can you do this for all archs ? My limited testing shows
that it (moving cpu_exec_init from instance_init to realize) works for
sPAPR PowerPC too, but not sure about other targets.

After there is consensus on the above two patches, I can do the
cpu_index to bitmap changes.

Regards,
Bharata.