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

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

On Wed, Mar 18, 2015 at 11:49:59AM +1100, David Gibson wrote:
> On Fri, Mar 13, 2015 at 05:26:36PM +0530, 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.
> 
> How does this interact with the tweaking of cpu indexes that spapr
> does in order to configure the guest SMT mode on POWER7 and POWER8
> systems?

I am not changing the mapping of cpu_index to cpu_dt_id. So nothing
should be change. Can you please point me to the piece of tweaking code
that you are referring to above ?

Regards,
Bharata.