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

[David Gibson]

On Fri, Mar 13, 2015 at 05:26:36PM +0530, 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.

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?

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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