Messages in this thread | | | Date | Wed, 1 May 2019 18:00:53 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH] RISC-V: Add an Image header that boot loader can parse. |
| |
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@wdc.com 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@wdc.com> > > > --- > > > 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.
| |