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Re: [Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with spec

From: David Gibson
Subject: Re: [Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with specified addr
Date: Mon, 20 Jul 2015 15:01:05 +1000
User-agent: Mutt/1.5.23 (2014-03-12)

On Fri, Jul 17, 2015 at 01:56:40PM +0200, Andrew Jones wrote:
> I've started playing with adding ppc support to kvm-unit-tests,
> using spapr for the machine model. I wanted to link the unit test
> at 0x400000 to match qemu's load address, making the unit test
> startup code simpler, but ended up with 0x800000 instead, due to
> how translate_kernel_address works. The translation makes sense
> for how Linux kernels are linked (always at 0xc0000000 or
> 0xc000000000000000), but for the unit test case we need to avoid
> adding the offset.
> Signed-off-by: Andrew Jones <address@hidden>
> ---
> Big RFC because I don't know if the "always at 0xc..." statement
> is 100% true for Linux, nor if this patch would break other stuff...

Yeah, I'm pretty dubious about this too, especially since I don't
entirely grasp what the load_elf() translation function is all about

At least 32-bit Linux on powerpc can have a different link offset -
it's actually a config option.  I can't see any practical purpose for
a link address that has any bits in the 0xfffffff mask though.

That said, I suspect making your unit test assume a fixed load address
may not be the best idea - qemu or SLOF could change in future to move
things about, so it might be more robust to have your test copy itself
to address it wants to be at before executing.

>  hw/ppc/spapr.c | 3 +++
>  1 file changed, 3 insertions(+)
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index c1cbf3387ae0c..4f1548f5168db 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -832,6 +832,9 @@ static void spapr_finalize_fdt(sPAPRMachineState *spapr,
>  static uint64_t translate_kernel_address(void *opaque, uint64_t addr)
>  {
> +    if ((addr & 0x0fffffff) == addr) {
> +        return addr;
> +    }
>      return (addr & 0x0fffffff) + KERNEL_LOAD_ADDR;
>  }

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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