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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, 16 Mar 2016 16:11:35 +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.

That's right.

> 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 ?

Ah..  My first guess would be that it's ok; since IIUC PowerVM doesn't
care where the LMBs are removed from, removing them starting from the
last place we removed something should be as good as anywhere.

But it's possible there's some issue I haven't considered.

> 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.

Ok.  It seems like it ought to be fairly straightforward, but I don't
know the drmgr code, so..

It would certainly be useful if Nathan could chime in on this.

> 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 ?

That's.. a good question.  I guess it means avoiding another interface
and a pseudo-device at least.

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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