Re: VW ELF loader

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jul 06, 2026 at 02:37:48PM +0530, Utkarsh Verma wrote:
> +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.

Good luck, but I'm afraid I no longer have any interest in, or time to
commit to looking at ppc qemu stuff.

> 
> 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
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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