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Re: [Qemu-ppc] [RFC PATCH v2] spapr: Support ibm, dynamic-memory-v2 prop

From: Bharata B Rao
Subject: Re: [Qemu-ppc] [RFC PATCH v2] spapr: Support ibm, dynamic-memory-v2 property
Date: Tue, 10 Apr 2018 09:45:21 +0530
User-agent: Mutt/1.9.2 (2017-12-15)

On Tue, Apr 10, 2018 at 01:02:45PM +1000, David Gibson wrote:
> On Mon, Apr 09, 2018 at 11:55:38AM +0530, Bharata B Rao wrote:
> > The new property ibm,dynamic-memory-v2 allows memory to be represented
> > in a more compact manner in device tree.
> I still need to look at this in more detail, but to start with:
> what's the rationale for this new format?
> It's more compact, but why do we care?  The embedded people always
> whinge about the size of the deivce tree, but I didn't think that was
> really a concern with PAPR.

Here's a real example of how this has affected us earlier:

SLOF's CAS FDT buffer size was initially 32K, was changed to 64k to
support 1TB guest memory and again changed to 2MB to support 16TB guest

With ibm,dynamic-memory-v2 we are less likely to hit such scenarios.

Also, theoretically it should be more efficient in the guest kernel
to handle LMB-sets than individual LMBs.

We aren't there yet, but I believe grouping of LMBs should eventually
help us do memory hotplug at set (or DIMM) granularity than at individual
LMB granularity (Again theoretical possibility)


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