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

From: Bharata B Rao
Subject: Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support
Date: Tue, 26 Apr 2016 10:39:23 +0530
User-agent: Mutt/1.5.23 (2014-03-12)

On Mon, Apr 25, 2016 at 11:20:50AM +0200, Igor Mammedov wrote:
> On Wed, 16 Mar 2016 10:11:54 +0530
> Bharata B Rao <address@hidden> 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.
> [...]
> > 
> > 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 ?
> Sure ballooning can complement memory hotplug but turning it on would
> effectively reduce hotplug to balloning as it would enable overcommit
> capability instead of hard partitioning pc-dimms provides. So one
> could just use ballooning only and not bother with hotplug at all.
> On the other hand memory hotplug/unplug (at least on x86) tries
> to model real hardware, thus removing need in paravirt ballooning
> solution in favor of native guest support.

Thanks for your views.

> PS:
> Guest wise, currently hot-unplug is not well supported in linux,
> i.e. it's not guarantied that guest will honor unplug request
> as it may pin dimm by using it as a non migratable memory. So
> there is something to work on guest side to make unplug more
> reliable/guarantied.

In the above scenario where the guest doesn't allow removal of certain
parts of DIMM memory, what is the expected behaviour as far as QEMU
DIMM device is concerned ? I seem to be running into this situation
very often with PowerPC mem unplug where I am left with a DIMM device
that has only some memory blocks released. In this situation, I would like
to block further unplug requests on the same device, but QEMU seems
to allow more such unplug requests to come in via the monitor. So
qdev won't help me here ? Should I detect such condition from the
machine unplug() handler and take required action ?

On x86, if some pages are offlined and subsequently other pages couldn't
be offlined, then I see the full DIMM memory size remaining
with the guest. So I infer that on x86, QEMU memory unplug either
removes full DIMM or nothing. Is that understanding correct ?


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