[RFC PATCH 2/2] ARM: SMP support for mach-virt

[Russell King - ARM Linux]

On Tue, Dec 04, 2012 at 01:40:10PM +0000, Will Deacon wrote:
> On Tue, Dec 04, 2012 at 01:33:26PM +0000, Russell King - ARM Linux wrote:
> > The memory that these 'offline' CPUs is executing then gets overwritten,
> > and that's game over for those CPUs.
> 
> That's not strictly true. The device-tree passed to the kernel should have a
> /memreserve/ entry for the SMP pen to avoid exactly this scenario. In real
> hardware, this still sucks because you have spinning CPUs burning up power
> but that's not such a problem with a virtual platform.

Umm.  So let's see.  If I'm running v3.6 stock kernel and want to kexec
into a v3.7 stock kernel.  The SMP pen is part of the v3.6 kernel, which
will be located at 32K into the RAM.  The v3.7 kernel will also want to
occupy the same place.  At some point you have to overwrite the v3.6
kernel with the v3.7 kernel image.

That happens _before_ the DT has been parsed, so any memreserve stuff
will be ignored.  And it's at that point that your "offline" secondary
CPUs will have their instructions overwritten.

That's fine if the pen ends up being at the same place but that's not
something we guarantee.