Re: [Qemu-devel] Allow ARMv7M to be started without a kernel

[Rob Landley <rob@landley.net>]

On 05/09/2011 10:50 AM, Peter Maydell wrote:
> On 9 May 2011 16:11, Alexander Graf <agraf@suse.de> wrote:
> [about -kernel, unless I've got confused]
>> The issue is that this is not how it works on real hardware. Grub won't just
>> load a vmlinux file and boot it. I'm not even sure how much exactly the
>> early entry code handles in Linux before it jumps to the ELF entry point.
>>
>> Either way, if you get something rolling that also ensures that it fails
>> when it's an ELF file that's not Linux, I'd be very open to it :).
> 
> If we do that we need to document what the new way of doing "just load
> and jump to the entry point of my not-a-linux-kernel ELF image" is; at
> the moment for ARM that use case is supported by -kernel (the code
> specifically handles ELF images as not-kernels), so changing that would
> be a back-compatibility break...

Arm doesn't need nearly as much setup as x86, some boards just map flash
at the physical start address with a jump straight to the kernel entry
point.

On arm there's no legacy start mode, meaning no 16->32 bit transfer
requiring mmu initialization as part of the setup.  (I believe arm
starts with a 1-1 virtual/physical mapping in the absence of initialized
page tables, or something like that.)  The big black magic thing
coreboot/bios and uboot do is DRAM refresh, which QEMU simply doesn't
care about: we map dram from the host and it gets refreshed at that level.

So this behavior may actually be correct for Linux in the absence of a
device tree (which just means initializinng a register to point to it on
ppc, dunno what ARM does).

And if you feed in an "-append" option to set a kernel command line,
then you know it's a linux kernel.  If we explicitly ask for more setup,
then you know to do more setup.  (I think that bit still needs to be
written, I have to go read the code...)

Rob