Re: [Qemu-devel] [RFC 1/3] using CPUMASK bitmaps to calculate cpu index

[address@hidden]

On Thu, 2014-05-22 at 15:26 +0200, Igor Mammedov wrote: 
> On Tue, 13 May 2014 18:08:47 +0800
> Chen Fan <address@hidden> wrote:
> 
> > instead of seeking the number of CPUs, using CPUMASK bitmaps to
> > calculate the cpu index, also would be a gread benefit to remove
> > cpu index.
> How would it help to remove cpu_index?
I think the cpu_index is just a CPU counter，if we remove one sparse CPU
by setting corresponding bit to zero. then adding a new one we could
easily get this vacancy bit by find_first_zero_bit() rather than get a 
duplicate index by calculating the number of CPU.

> What if there is only one CPU at start nad there is a need to add
> CPU with cpu_index==10?
> 
> Bitmap hardly changes here anything, it's just another way of
> doing:
>    cpu_index = 0;
>    CPU_FOREACH(some_cpu) {
>        cpu_index++;
>    }
> 
> 
> What would help however is replacing cpu_index at read points with
> CPUClass->get_arch_id()
> 
> Then targets that need sparse index could override default
> get_arch_id() to suite their needs and use their own properties
> to set arch_id value.
> 
> Caution: this change would cause migration breakage for target-i386,
> so x86_cpu_get_arch_id() should take care about it when used with old
> machine types.
yes, but for migration, I think we should need to override the cpu index
in function vmstate_register(NULL, cpu_index, &vmstate_cpu_common, cpu)
with an unique instance_id while hot-add/hot-remove CPU arbitrarily.
maybe this unique instance_id can be 'apic-id'.

Thanks,
Chen

> 
> > 
> > Signed-off-by: Chen Fan <address@hidden>
> > ---
> >  exec.c                  | 9 ++++-----
> >  include/qom/cpu.h       | 9 +++++++++
> >  include/sysemu/sysemu.h | 7 -------
> >  3 files changed, 13 insertions(+), 12 deletions(-)
> > 
> > diff --git a/exec.c b/exec.c
> > index cf12049..2948841 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -473,16 +473,15 @@ 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_index = find_first_zero_bit(cc->cpu_present_mask,
> > +                                    MAX_CPUMASK_BITS);
> > +    set_bit(cpu_index, cc->cpu_present_mask);
> > +
> >      cpu->cpu_index = cpu_index;
> >      cpu->numa_node = 0;
> >      QTAILQ_INIT(&cpu->breakpoints);
> > diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> > index df977c8..b8f46b1 100644
> > --- a/include/qom/cpu.h
> > +++ b/include/qom/cpu.h
> > @@ -70,6 +70,13 @@ typedef void (*CPUUnassignedAccess)(CPUState *cpu, 
> > hwaddr addr,
> >  
> >  struct TranslationBlock;
> >  
> > +/* The following shall be true for all CPUs:
> > + *   cpu->cpu_index < max_cpus <= MAX_CPUMASK_BITS
> > + *
> > + * Note that cpu->get_arch_id() may be larger than MAX_CPUMASK_BITS.
> > + */
> > +#define MAX_CPUMASK_BITS 255
> > +
> >  /**
> >   * CPUClass:
> >   * @class_by_name: Callback to map -cpu command line model name to an
> > @@ -142,6 +149,8 @@ typedef struct CPUClass {
> >      const struct VMStateDescription *vmsd;
> >      int gdb_num_core_regs;
> >      const char *gdb_core_xml_file;
> > +
> > +    DECLARE_BITMAP(cpu_present_mask, MAX_CPUMASK_BITS);
> >  } CPUClass;
> >  
> >  #ifdef HOST_WORDS_BIGENDIAN
> > diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> > index ba5c7f8..04edb8b 100644
> > --- a/include/sysemu/sysemu.h
> > +++ b/include/sysemu/sysemu.h
> > @@ -134,13 +134,6 @@ extern QEMUClockType rtc_clock;
> >  
> >  #define MAX_NODES 64
> >  
> > -/* The following shall be true for all CPUs:
> > - *   cpu->cpu_index < max_cpus <= MAX_CPUMASK_BITS
> > - *
> > - * Note that cpu->get_arch_id() may be larger than MAX_CPUMASK_BITS.
> > - */
> > -#define MAX_CPUMASK_BITS 255
> > -
> >  extern int nb_numa_nodes;
> >  extern uint64_t node_mem[MAX_NODES];
> >  extern unsigned long *node_cpumask[MAX_NODES];
>