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Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH v2] spapr: Support ibm, dynamic-m

From: David Gibson
Subject: Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH v2] spapr: Support ibm, dynamic-memory-v2 property
Date: Wed, 11 Apr 2018 14:15:53 +1000
User-agent: Mutt/1.9.2 (2017-12-15)

On Wed, Apr 11, 2018 at 01:56:14PM +1000, Alexey Kardashevskiy wrote:
1;5002;0c> On 10/4/18 1:02 pm, 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.
> Well, booting a guest with 500 pci devices (let's say emulated e1000 or
> virtio-net) creates >20sec delay in the guest while it is fetching the
> device tree in early setup, property by property, I even have a patch for
> pseries guest to get FDT as a whole blob from SLOF. It is not The Problem
> but still annoying.

Right, but 500 PCI devices is not this case.  Sounds like the slowness
there is mostly from having lots of nodes and properties and therefore
lots of individual OF calls.  The old dynamic memory format is still
just one big property, so it shouldn't have nearly the same impact.

Besides, Bharata pretty much convinced me already with his other reasons.

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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