Re: [Qemu-devel] [PATCH 1/2] Allow QEMUMachine to override reset sequencing

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2012-08-01 at 21:37 -0500, Anthony Liguori wrote:
> 
> But all of the DT initialization stuff that is leading to this
> discussion in the first place is a gross hack to make a PV
> architecture
> work that took far too many short cuts.
> 
> Building a DT in memory representing hardware instead of making things
> discoverable is not how hardware works. 

It depends :-)

Take a Power7 machine for example... the system & processor are actually
initialized by the service processor which loads some kind of similar
data structure into memory before starting the initial firmware :-)

So from the P7 itself point of view, it starts with a memory based data
structure pre-build.

Also some processors have a pretty much turing complete "POR engine"
which executes code from some kind of EEPROM to initialize the system,
cores & busses on reset, which can involve writing things to memory as
well (though it generally doesn't).

>  This sort of stuff is either
> (1) hard coded in a firmware/flashrom or (2) built dynamically in
> firmware.  Let's not pretend like we're doing this because it's needed
> for real hardware.

Doesn't matter, we do things like -kernel which means pre-loading the
kernel from qemu, even on x86. That doesn't match real HW either, but
it's convenient to have. But overall there are real HW reasons to
control the reset as well so the hook makes sense both ways.

Cheers,
Ben.