Re: [Qemu-devel] dynamic sysbus instantiation and load_dtb implementation

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Oct 24, 2014 at 02:38:39PM +0200, David Gibson wrote:
> On Thu, Oct 23, 2014 at 01:26:08PM +0200, Alexander Graf wrote:
> > 
> > 
> > On 23.10.14 13:24, Peter Maydell wrote:
> > > On 23 October 2014 12:23, Alexander Graf <agraf@suse.de> wrote:
> > >> On 23.10.14 12:19, Ard Biesheuvel wrote:
> > >>> The reason for this change was that, before, the DTB would only be
> > >>> generated once, and after a reset, the machine would go through the
> > >>> kernel boot protocol as before but the DTB pointer would point to
> > >>> garbage. Any idea how ppc deals with this? Do they recreate the device
> > >>> tree after each reset?
> > >>
> > >> Yes, we regenerate the device tree on each reset.
> > > 
> > > Any particular reason? Surely it's always the same...
> > 
> > We have the code in place anyway, it's not a performance critical code
> > path and putting it into a rom would be a waste of RAM, as it'd keep yet
> > another copy of something we can easily regenerate.
> > 
> > It's a matter of personal preference I guess.
> 
> The "pseries" machine actually uses an odd hybrid.  We create a
> "template" device tree with the common portions during early init.
> That's stored permanently, but is not guest visible.
> 
> At reset time we augment the template with information which could
> very from one boot to another, then copy it into RAM for the SLOF
> firmware to read.
> 
> It's not particularly efficient, but really, who cares.  It's once per
> resent, and we're generally talking at most a few dozen kiB of device
> tree.

Oh, also, because we potentially have hotplug of VIO devices, it's
*not* necessarily the same every time.

-- 
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_!
http://www.ozlabs.org/~dgibson

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