Re: VW ELF loader

[Utkarsh Verma <uverma@linux.ibm.com>]

+Thomas, Narayana, Shiva, Harsh - FYI

Hi Alexey, David and Paolo,
Reviving this thread as would like to add the disk boot support by 
loading GRUB.

On 2/1/20 7:09 PM, Alexey Kardashevskiy wrote:
> Hi!
>
> In my effort to "kill SLOF" (the PPC pseries guest firmware), I 
> proceeded to the stage when QEMU needs to load GRUB from the disk. The 
> current workaround is to read it from qcow2, save in a file and then 
> call load_elf(). Not nice.
>
> 2 problems with that.
>
> 1. when load_elf calls address_space_write() - I need to know where 
> and how much RAM was used to mark this memory "used" for the OF client 
> interface (/memory@0/available FDT property). So I'll need "preload()" 
> hook.
>
> 2. (bigger) GRUB comes from PReP partition which is 8MB. 
> load_elf{32|64} consumes filename, not a memory pointer nor a 
> "read_fn" callback - so I thought I need a "read_fn" callback.
>
> And then I discovered that load_elf actually maps the passed file. And 
> here I got lost.
>
> Why does not load_elf just map the entire file and parse the bits? It 
> still reads chunks with seek+read and then it maps the file in a loop 
> potentially multiple times - is this even correct? Passing "fd" around 
> is weird.
>
> Why ROMs are different from "-kernel"?
>
> If I want to solve 1 and 2 of my problem, should I just cut-n-paste 
> load_elf and tweak bits rather then add more parameters to already 
> 15-parameters long prototypes?
> Or I could read GRUB from qcow2 into the memory and change the rest to 
> parse ELF from memory (mapped from a ELF file or read from qcow2)? 
To rectify the mentioned issues, would like to implement a custom ELF 
loader for ppc that can load the elf from the buffer and claim the 
memory for each program header segment.

It was attempted earlier in v6 (Patch 6/6) and v7 (Patch 5/5) , but 
later dropped for reasons unknown to me:
v6: 
https://lore.kernel.org/qemu-devel/20200203032943.121178-1-aik@ozlabs.ru/
v7: https://lore.kernel.org/qemu-devel/20200220061622.15064-1-aik@ozlabs.ru/

I believe disk boot is a worthwhile addition to VOF. It would make the 
pseries machine type fully usable without SLOF for the most common case: 
booting from a distro qcow2 image with a PReP partition containing GRUB.

>
>
>
> Thanks,
>
> ps. VW == very weird, indeed :)
>

Regards,
Utkarsh Verma