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

[Rob Landley <rob@landley.net>]

On 05/09/2011 09:11 AM, Alexander Graf wrote:
>> C) requires more research, because I have to make sure the entry point
>> is either doing the 16->32 (or 64) bit startup dance or that it's being
>> launched in the right mode (which the bios isn't doing), but vmlinux
>> doesn't need to be decompressed and copied so it's just making sure
>> we're expecting the right layer of stuff.  Which means reading through
>> arch/x86/boot to see what the vmlinux->bzImage packaging is adding, I
>> suppose.
> 
> The issue is that this is not how it works on real hardware.

Real hardware doesn't have the -kernel option.  On x86 that option is
currently patching the rdev area after loading the blob into DRAM.

> Grub won't just load a vmlinux file and boot it.

The -kernel option doesn't load grub, it loads a _kernel_.  -kernel is a
linux-specific bootloader.  It doesn't chainload other bootloaders.

> 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 it's an ELF file that's not Linux, I can ensure the result won't boot
Linux.  It's guaranteed to fail without me doing anything.

I don't understand your objection.  (The option is called -kernel.  It
does what it says on the tin.  If you feed it something that isn't a
kernel, which you can do now, this is pilot error.)

By the way, I just did "qemu -kernel qemu-img" an the emulated vga
screen was very colorful.  Feeding it qemu-io gave a different pattern
of vga text mode garbage.  It's happily loading an elf file _now_, and
spinning its little CPU on the resulting garbage...

>> But "-kernel vmlinux" is not currently at the top of my todo list, lemme
>> finish studying the v9fs code first...
>>
>>> Point being, the handling of -bios is board-specific (not even
>>> architecture-specific) and thus inconsistent.
>> So, currently, is -kernel.  Having it accept vmlinux on more platforms
>> would make it more consistent.  (Having -bios accept that too sounds
>> great, but is orthogonal to what I'm trying to do.)
>>
>>> Your points above - page
>>> tables, TLB entries etc. - only apply to -kernel but not to -bios, that
>>> should give us a bare-metal machine.
>> In 16 bit mode.
> 
> Depends on the machine. On PPC e500 for example, even with -bios we have
> to set up TLB entries, but those are at least defined by the spec :).

Yes, -kernel has to do setup work before starting the kernel.  This is
true now, it remains true if the kernel is in a different file format.
-kernel is a linux bootloader, which is not the same thing as the -bios
option.

Rob