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

[Atish Patra <atish.patra@wdc.com>]

On 5/1/19 10:02 AM, Anup Patel wrote:
> On Wed, May 1, 2019 at 10:14 PM Karsten Merker <merker@debian.org> wrote:
>>
>> 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).
>>>
>>> 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).
> 

Karsten,

> The only difference compared to ARM64 is the values of code0, code1
> and res5 fields.
> 
> As-per PE/COFF, the 32bit value at offset 0x3c tells us offset of PE/COFF
> header in image.
> 
> For more details refer,
> https://en.wikipedia.org/wiki/Portable_Executable
> https://en.wikipedia.org/wiki/Portable_Executable#/media/File:Portable_Executable_32_bit_Structure_in_SVG_fixed.svg
> 
> For both ARM64 header and RISC-V image header, is actually the
> "DOS header" part of PE/COFF format.
> 
> This patch only adds "DOS header" part of PE/COFF format. Rest of
> the PE/COFF header will be added when add EFI support to Linux
> RISC-V kernel.
> I think Anup answered your question. The original plan was to add EFI 
specific stuff in EFI support patch. That includes adjusting the PE/COFF 
offset at 0x3c and adding the "MZ" value for code0 if EFI is enabled.

In hindsight, I think it created more confusion. I will update the 
"riscv_image_header" structure to put PE/COFF offset(0x3c) at right 
place in v2 patch to avoid further confusion.

"MZ" value part should be added once EFI is enabled.

I will update the comments on riscv/include/asm/image.h as well to 
clarify more.

Regards,
Atish
> Regards,
> Anup
>