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Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support

From: David Gibson
Subject: Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support
Date: Wed, 23 Mar 2016 14:22:30 +1100
User-agent: Mutt/1.5.24 (2015-08-30)

On Wed, Mar 16, 2016 at 10:11:54AM +0530, Bharata B Rao wrote:
> On Wed, Mar 16, 2016 at 12:36:05PM +1100, David Gibson wrote:
> > On Tue, Mar 15, 2016 at 10:08:56AM +0530, Bharata B Rao wrote:
> > > Add support to hot remove pc-dimm memory devices.
> > > 
> > > Signed-off-by: Bharata B Rao <address@hidden>
> > 
> > Reviewed-by: David Gibson <address@hidden>
> > 
> > Looks correct, but again, needs to wait on the PAPR change.
> > 
> > Have you thought any further on the idea of sending an index message,
> > then a count message as an interim approach to fixing this without
> > requiring a PAPR change?
> Removal by index and removal by count are valid messages by themselves
> and drmgr would go ahead and start the removal in reponse to those
> calls. IIUC, you are suggesting that lets remove one LMB by index in
> response to 1st message and remove (count -1) LMBs from where the last
> removal was done in the previous message.

Yes, that's the idea.

> Since the same code base of powerpc-utils works on PowerVM too, I am not
> sure if such an approach would impact PowerVM in any undesirable manner.
> May be Nathan can clarify ?

Heard anything from Nathan?  I don't really see how it would be bad
under PowerVM.  Under PowerVM it generally doesn't matter which LMBs
you remove, right?  So removing the ones immediately after the last
one you removed should be as good a choice as any.

> I see that this can be done, but the changes in drmgr code specially the
> code related to LMB list handling/removal can be non-trivial. So not sure
> if the temporary approach is all that worth here and hence I feel it is better
> to wait and do it the count-indexed way.

Really?  drmgr is already scanning LMBs to find ones it can remove.
Seeding that scan with the last removed LMB shouldn't be too hard.

> While we are here, I would also like to get some opinion on the real
> need for memory unplug. Is there anything that memory unplug gives us
> which memory ballooning (shrinking mem via ballooning) can't give ?

Hmm.. that's an interesting question.  

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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