Re: [PATCH qemu] spapr: Use address from elf parser for kernel address

[Fabiano Rosas <farosas@linux.ibm.com>]

Alexey Kardashevskiy <aik@ozlabs.ru> writes:

> tl;dr: This allows Big Endian zImage booting via -kernel + x-vof=on.
>
> QEMU loads the kernel at 0x400000 by default which works most of
> the time as Linux kernels are relocatable, 64bit and compiled with "-pie"
> (position independent code). This works for a little endian zImage too.
>
> However a big endian zImage is compiled without -pie, is 32bit, linked to
> 0x4000000 so current QEMU ends up loading it at
> 0x4400000 but keeps spapr->kernel_addr unchanged so booting fails.
>
> This uses the kernel address returned from load_elf().
> If the default kernel_addr is used, there is no change in behavior (as
> translate_kernel_address() takes care of this), which is:
> LE/BE vmlinux and LE zImage boot, BE zImage does not.
> If the VM created with "-machine kernel-addr=0,x-vof=on", then QEMU
> prints a warning and BE zImage boots.

I think we can fix this without needing a different command line for BE
zImage (apart from x-vof, which is a separate matter).

If you look at translate_kernel_address, it cannot really work when the
ELF PhysAddr is != 0. We would always hit this sort of 0x4400000 issue,
so if we fix that function like this...

static uint64_t translate_kernel_address(void *opaque, uint64_t addr)
{
    SpaprMachineState *spapr = opaque;

    return addr ? addr : spapr->kernel_addr;
}

...then we could always use the ELF PhysAddr if it is different from 0
and only use the default load addr if the ELF PhysAddr is 0. If the user
gives kernel_addr on the cmdline, we honor that, even if puts the kernel
over the firmware (we have code to detect that).

> @@ -2988,6 +2990,12 @@ static void spapr_machine_init(MachineState *machine)
>              exit(1);
>          }
>  
> +        if (spapr->kernel_addr != loaded_addr) {

This could be:

        if (spapr->kernel_addr == KERNEL_LOAD_ADDR &&
	    spapr->kernel_addr != loaded_addr) {

So the precedence would be:

1- ELF PhysAddr, if != 0. After all, that is what it's for. BE zImage
   falls here;
    
2- KERNEL_LOAD_ADDR. Via translate_kernel_address, LE/BE vmlinux fall
   here;

3- kernel_addr. The user is probably hacking something, just use what
   they gave us. QEMU will yell if they load the kernel over the fw.

> +            warn_report("spapr: kernel_addr changed from 0x%lx to 0x%lx",
> +                        spapr->kernel_addr, loaded_addr);
> +            spapr->kernel_addr = loaded_addr;
> +        }
> +
>          /* load initrd */
>          if (initrd_filename) {
>              /* Try to locate the initrd in the gap between the kernel