Re: [Qemu-devel] [PATCH 3/3] PPC PReP: can run without bios image

["Andreas Färber" <afaerber@suse.de>]

Am 04.04.2013 18:17, schrieb Fabien Chouteau:
> On 04/04/2013 02:43 PM, Peter Maydell wrote:
>> On 4 April 2013 12:53, Andreas Färber <afaerber@suse.de> wrote:
>>> Alex, isn't ARM running without -bios? Instead of a firmware blob it has
>>> some hardcoded firmware'ish instructions in the loader code.
>>
>> Varies from board to board, but yes, generally we have a trivial
>> bootloader (which on uniprocessor machines doesn't actually run
>> guest code, it just sets registers and memory up to jump to the
>> kernel).
>>
>>> For PReP, Fabien has not stated what his use case actually is (in
>>> particular which hardware?), so it's hard for me to comment on what the
>>> hardware actually does and I thus won't accept random changes just
>>> because they happen to be in Leon3 code. There's nothing conceptually
>>> wrong with loading ELF code so I'm positive we will find a solution to
>>> accommodate all use cases in some way. :)
>>
>> ARM also lets you pass an ELF file to -kernel which it treats
>> as "just pull this blob into RAM and jump to its entrypoint".
> 
> That's what we do for almost all the targets we are using at AdaCore
> (leon, leon3, PReP(602), wrSbc8349(e300), wrSbc8548(e500v2),
> lm3s(arm-M3), TMS570(arm-R4F)).
> 
>> This is useful for 'bare metal' type test cases (equivalent
>> of dumping a file in over JTAG).
> 
> Exactly, maybe I have to explain how we use QEMU here:
> 
> The Ada language provides run-time features (tasking, timing services,
> protected objects, etc...). These can be implemented on top of an OS
> (Windows, Linux, Solaris, vxWorks, etc...). There's also the Ravenscar
> profile, which restrict the language to a subset of run-time features
> suitable for embedded and safety-critical tasks.
> 
> In that case there's no need for and OS, but the program itself can be a
> simple kernel (scheduling, interrupts, timing services...) running on
> bare metal. The program will also do the initialization of the board, so
> there's no need for bootloader/firmware.

Okay, so that's -bios for PReP at least and my patch adds the missing
ELF support, with fallback to current blob loading.

> We run hundreds of thousands of tests on QEMU each days, on all the
> guest platforms above and on Windows and Linux hosts. These tests are
> very short programs that usually run for less than 2 secs.
> 
> In that context having a boot loader that initialize the board and load
> the program from network or disk is not very interesting, it's time
> consuming and it's complicated. Our compiler builds and ELF file, it's
> easier to run it right away.
> 
>> The UI is all wrong, though:
>> -kernel should always mean "load a Linux kernel" and we should
>> have some other way (ideally a cross-architecture way) of saying
>> "just load this binary blob and start it". (-bios isn't that
>> because -bios tends to (a) mean different things on different
>> boards and (b) mean 'put this in flash or whatever' rather than
>> 'dump stuff in RAM and go'.)
>>
> 
> I think -kernel works fine for all architecture. Linux is not the only
> kernel available ;)

I fear that machines have hardcoded some Linux'ish assumptions of where
things get placed, including -append arguments, and what the state the
hardware is in. ;)

-kernel is supposed to be a fast way of loading a kernel, bypassing the
boot loader the hardware usually has, whereas -bios loads something to
baremetal hardware without changing registers from the hardware reset
defaults.

Andreas

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