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

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

On Wed, Mar 25, 2015 at 02:23:29PM +1100, David Gibson wrote:
> 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 <bharata@linux.vnet.ibm.com>
> > ---
> >  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.

It is called in subsequent patch. If you are suggesting that a function
shouldn't be defined in a patch where it not used, I can move this
down to the other patch.

Regards,
Bharata.