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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: Fri, 29 Apr 2016 20:11:40 +1000
User-agent: Mutt/1.5.24 (2015-08-30)

On Fri, Apr 29, 2016 at 08:45:37AM +0200, Thomas Huth wrote:
> On 29.04.2016 05:24, David Gibson wrote:
> > On Tue, Apr 26, 2016 at 04:03:37PM -0500, Michael Roth wrote:
> ...
> >> In the case of pseries, the DIMM abstraction isn't really exposed to
> >> the guest, but rather the memory blocks we use to make the backing
> >> memdev memory available to the guest. During unplug, the guest
> >> completely releases these blocks back to QEMU, and if it can only
> >> release a subset of what's requested it does not attempt to recover.
> >> We can potentially change that behavior on the guest side, since
> >> partially-freed DIMMs aren't currently useful on the host-side...
> >>
> >> But, in the case of pseries, I wonder if it makes sense to maybe go
> >> ahead and MADV_DONTNEED the ranges backing these released blocks so the
> >> host can at least partially reclaim the memory from a partially
> >> unplugged DIMM?
> > 
> > Urgh.. I can see the benefit, but I'm a bit uneasy about making the
> > DIMM semantics different in this way on Power.
> > 
> > I'm shoehorning the PAPR DR memory mechanism into the qemu DIMM model
> > was a good idea after all.
> Ignorant question (sorry, I really don't have much experience yet here):
> Could we maybe align the size of the LMBs with the size of the DIMMs?
> E.g. make the LMBs bigger or the DIMMs smaller, so that they match?

Um... maybe.  DIMMs don't have to all be the same size, whereas LMBs
do, but maybe we can work around that.

In theory we could increase the LMB size, but I'd be pretty worried
that guests might not cope with a setup so different from what PowerVM
gives us.

Decreasing the DIMMs to LMB size should certainly work in theory, but
could be very painful from the user point of view to have to add a
memory block for every 256MiB.

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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