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

From: Thomas Huth
Subject: Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support
Date: Fri, 29 Apr 2016 08:45:37 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.0

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?


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