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Re: [Qemu-devel] [PATCH RFC] spapr: disintricate core-id from DT semanti
Re: [Qemu-devel] [PATCH RFC] spapr: disintricate core-id from DT semantics
Wed, 27 Jul 2016 12:06:17 +0200
Le Mon, 25 Jul 2016 16:01:31 +1000,
David Gibson <address@hidden> a écrit :
> 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.
I was too busy packing for holidays already :) and I think the consolidation
work can be addressed in followup patches.
Cheers from Brittany, under the rain.
> > Other uses for core-id includes: compute a stable cpu_index (which
> > allows random order hotplug/unplug without breaking migration) and
> > NUMA.
> > 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
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