Re: [PATCH] RISC-V: Add an Image header that boot loader can parse.

[Mark Rutland]

On Mon, Apr 29, 2019 at 10:42:40PM -0700, Atish Patra wrote:
> On 4/29/19 4:40 PM, Palmer Dabbelt wrote:
> > On Tue, 23 Apr 2019 16:25:06 PDT (-0700), atish.patra@xxxxxxx wrote:
> > > Currently, last stage boot loaders such as U-Boot can accept only
> > > uImage which is an unnecessary additional step in automating boot flows.
> > > 
> > > Add a simple image header that boot loaders can parse and directly
> > > load kernel flat Image. The existing booting methods will continue to
> > > work as it is.
> > > 
> > > Tested on both QEMU and HiFive Unleashed using OpenSBI + U-Boot + Linux.
> > > 
> > > Signed-off-by: Atish Patra <atish.patra@xxxxxxx>
> > > ---
> > >   arch/riscv/include/asm/image.h | 32 ++++++++++++++++++++++++++++++++
> > >   arch/riscv/kernel/head.S       | 28 ++++++++++++++++++++++++++++
> > >   2 files changed, 60 insertions(+)
> > >   create mode 100644 arch/riscv/include/asm/image.h
> > > 
> > > diff --git a/arch/riscv/include/asm/image.h b/arch/riscv/include/asm/image.h
> > > new file mode 100644
> > > index 000000000000..76a7e0d4068a
> > > --- /dev/null
> > > +++ b/arch/riscv/include/asm/image.h
> > > @@ -0,0 +1,32 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +
> > > +#ifndef __ASM_IMAGE_H
> > > +#define __ASM_IMAGE_H
> > > +
> > > +#define RISCV_IMAGE_MAGIC	"RISCV"
> > > +
> > > +#ifndef __ASSEMBLY__
> > > +/*
> > > + * struct riscv_image_header - riscv kernel image header
> > > + *
> > > + * @code0:		Executable code
> > > + * @code1:		Executable code
> > > + * @text_offset:	Image load offset
> > > + * @image_size:		Effective Image size
> > > + * @reserved:		reserved
> > > + * @magic:		Magic number
> > > + * @reserved:		reserved
> > > + */
> > > +
> > > +struct riscv_image_header {
> > > +	u32 code0;
> > > +	u32 code1;
> > > +	u64 text_offset;
> > > +	u64 image_size;
> > > +	u64 res1;
> > > +	u64 magic;
> > > +	u32 res2;
> > > +	u32 res3;
> > > +};
> > 
> > I don't want to invent our own file format.  Is there a reason we can't just
> > use something standard?  Off the top of my head I can think of ELF files and
> > multiboot.
> 
> Additional header is required to accommodate PE header format. Currently,
> this is only used for booti command but it will be reused for EFI headers as
> well. Linux kernel Image can pretend as an EFI application if PE/COFF header
> is present. This removes the need of an explicit EFI boot loader and EFI
> firmware can directly load Linux (obviously after EFI stub implementation
> for RISC-V).

Adding the EFI stub on arm64 required very careful consideration of our
Image header and the EFI spec, along with the PE/COFF spec.

For example, to be a compliant PE/COFF header, the first two bytes of
your kernel image need to be "MZ" in ASCII. On arm64 we happened to find
a valid instruction that we could rely upon that met this requirement...

> > >   __INIT
> > >   ENTRY(_start)
> > > +	/*
> > > +	 * Image header expected by Linux boot-loaders. The image header data
> > > +	 * structure is described in asm/image.h.
> > > +	 * Do not modify it without modifying the structure and all bootloaders
> > > +	 * that expects this header format!!
> > > +	 */
> > > +	/* jump to start kernel */
> > > +	j _start_kernel

... but it's not clear to me if this instruction meets that requriement.

I would strongly encourage you to consider what you actually need for a
compliant EFI header before you set the rest of this ABI in stone.

On arm64 we also had issues with endianness, and I would strongly
recommend that you define how big/little endian will work ahead of time.
e.g. whether fields are always in a fixed endianness.

Thanks,
Mark.