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

[Karsten Merker]

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).
> 
> ARM64 follows the similar header format as well.
> https://www.kernel.org/doc/Documentation/arm64/booting.txt

Hello Atish,

the arm64 header looks a bit different (quoted from the
aforementioned URL):

  u32 code0;                    /* Executable code */
  u32 code1;                    /* Executable code */
  u64 text_offset;              /* Image load offset, little endian */
  u64 image_size;               /* Effective Image size, little endian */
  u64 flags;                    /* kernel flags, little endian */
  u64 res2      = 0;            /* reserved */
  u64 res3      = 0;            /* reserved */
  u64 res4      = 0;            /* reserved */
  u32 magic     = 0x644d5241;   /* Magic number, little endian, "ARM\x64" */
  u32 res5;                     /* reserved (used for PE COFF offset) */

What I am unclear about is in which ways a RISC-V PE/COFF header
differs from an arm64 one as the arm64 struct is longer than your
RISC-V header and for arm64 the PE offset field is in the last
field, i.e. outside of the area covered by your RISC-V structure
definition.  Can you perhaps explain this part in a bit more
detail or does anybody else have a pointer to a specification of
the RISC-V PE/COFF header format (I have found a lot of documents
about COFF in general, but nothing specific to RISC-V).

Regards,
Karsten
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