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Re: [Qemu-devel] [PATCH RFC] spapr: disintricate core-id from DT semanti

From: David Gibson
Subject: Re: [Qemu-devel] [PATCH RFC] spapr: disintricate core-id from DT semantics
Date: Mon, 25 Jul 2016 16:01:31 +1000
User-agent: Mutt/1.6.2 (2016-07-01)

On Fri, Jul 22, 2016 at 01:10:36PM +0200, Greg Kurz wrote:
> The goal of this patch is to have a stable core-id which does not depend
> on any DT related semantics, which involve non-obvious computations on
> modern PowerPC server cpus.
> With this patch, the DT core id is computed on-demand as:
>        (core-id / smp_threads) * smt
> where smt is the number of threads per core in the host.
> This formula should be consolidated in a helper since it is needed in
> several places.

It's a little odd you node this but don't do so.

> Other uses for core-id includes: compute a stable cpu_index (which
> allows random order hotplug/unplug without breaking migration) and
> Signed-off-by: Greg Kurz <address@hidden>
> ---
> It was first suggested here:
> https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg01727.html
> and as option 1) in the following discussion on IRC:
> <dwg> imammedo, basically the options are: 1) change core-ids to be
>  0, 1, .. n and compute cpu_index as core_id * threads + thread#, or
>  2) leave core-ids as they are and calculate cpu_index as
>  core-id / smt * threads + thread#
> It is based on David's ppc-for-2.7 branch (commit bb6268f35f457).
> It is lightly tested but I could at least do in-order
> hotplug/unplug.

I think this is basically the right approach, and I've applied to
ppc-for-2.7.  Here's my plan for what to do about all this id stuff:

  1. Merge this to ppc-for-2.7 (done)
  2. Assuming there are now show-stoppers in testing, send a pull
     request tomorrow
  3. Once this is merged, try to get Igor's series (or a respin of it)
     in ASAP.

I'm hoping that will give us good-enough hotplug for 2.7.

In the 2.8 timeframe, I want to:
  4. Disconnect KVM vcpu ID from dt_id, instead calculate it from (now
     stable) cpu_index
  5. Remove dt_id as a cpu field - instead just compute DT ids from
     the (now stable) cpu_index when we build the DT
  6. (for new machine type versions) Change DT ID assignment, so it
      no longer depends on kvmppc_smt_threads().  The current scheme
      means that migration between hosts with different native SMT
      values won't work, which is unfortunate.  I suspect there may be
      other problems with any real situation where that's the case,
      but nontheless it's a silly restriction.

Nice to have but bigger scope things for 2.8:
  7. Update archs to they *all* call cpu_exec_init() / cpu_exec_exit()
     at realize / unrealize time instead of init / finalize time.
  8. Update all archs and machines to use stable cpu_index

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_!

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