Re: [Qemu-devel] [PATCH 10/10] PPC: e500: Move to u-boot as firmware

[Alexander Graf]

On 23.01.2014, at 01:23, Scott Wood <address@hidden> wrote:

> On Mon, 2014-01-20 at 00:44 +0100, Alexander Graf wrote:
>> Almost all platforms QEMU emulates have some sort of firmware they can load
>> to expose a guest environment that closely resembles the way it would look
>> like on real hardware.
>> 
>> This patch introduces such a firmware on our e500 platforms. U-boot is the
>> default firmware for most of these systems and as such our preferred choice.
>> 
>> For now, it is able to expose the same functionality to the user as the 
>> direct
>> -kernel option was, just that it prints some nice messages beforehand.
>> 
>> However, if you abort the boot (press any key) or boot without -kernel you
>> are actually able to modify the boot environment, execute a networked boot
>> through the e1000 emulation and execute u-boot payloads.
> 
> Have you removed support for loading the kernel directly?  I thought you
> said you were only going to load U-Boot in the absence of -kernel.
> Given the absence of runtime services, and the presence of a
> standardized OS entry mechanism (ePAPR), inserting firmware into the
> process seems like a niche use case rather than something that should be
> the default, much less mandatory.

It's definitely not mandatory, but it makes things consistent. I'm not sure 
it's the better approach, but I figured we should definitely be able to go 
through the full u-boot cycle before we enter the guest kernel.

> 
>> +    /* Load u-boot (ELF) */
>> +    if (bios_name == NULL) {
>> +        bios_name = "u-boot.e500";
>> +    }
>> +    filename = qemu_find_file(QEMU_FILE_TYPE_BIOS, bios_name);
>> +
>> +    bios_size = load_elf(filename, NULL, NULL, &bios_entry, &loadaddr, NULL,
>> +                         1, ELF_MACHINE, 0);
>> +    if (bios_size < 0) {
>> +        /*
>> +         * Hrm. No ELF image? Try a uImage, maybe someone is giving us an
>> +         * ePAPR compliant kernel
>> +         */
> 
> ePAPR specifies ELF, not uImage (though QEMU does currently support
> ePAPR-ish loading of uImages, as U-Boot does) -- and you still seem to
> be trying to load firmware here, not a kernel.

Hrm. How do we load an ELF image when we have non-0 mapped memory? In fact, I'm 
wondering the same on uImages now that I think of it :).

So what this code does is it treats the "-bios" parameter the same as we used 
to treat -kernel, just in reverse probe order. So we check if -bios is an ELF 
image, if so, load it. If not, we load it as uImage and run it.

That means if you do -bios uImage you get a direct kernel boot, the same way we 
used to with -kernel before this patch set.

If we really want to make firmware only pop up when no -kernel is given (again, 
I'm not decided on that one yet), all we need to do is add this piece of code 
before the bios loader:

  if (args->kernel_filename) {
      /* Treat a -kernel passed kernel as firmware. This is possible because we 
don't have runtime services. */
      bios_name = args->kernel_filename;
  }

Then if we ever want to support both, all we need to do is replace the above 
piece of code with something that checks for a machine option on whether we 
want to skip firmware or not.


Alex