Re: [v4 PATCH] RISC-V: Add an Image header that boot loader can parse.

[Karsten Merker <merker@debian.org>]

On Tue, May 28, 2019 at 03:54:02AM +0000, Anup Patel wrote:
> > From: Troy Benjegerdes <troy.benjegerdes@sifive.com>
> > > On May 27, 2019, at 5:16 PM, Karsten Merker <merker@debian.org>
> > wrote:
> > >
> > > On Mon, May 27, 2019 at 04:34:57PM +0200, Ard Biesheuvel wrote:
> > >> On Fri, 24 May 2019 at 06:18, Atish Patra <atish.patra@wdc.com> wrote:
> > >>> Currently, the last stage boot loaders such as U-Boot can accept
> > >>> only uImage which is an unnecessary additional step in automating
> > >>> boot process.
> > >>>
> > >>> Add an image header that boot loader understands and boot Linux from
> > >>> flat Image directly.
> > >>>
> > >>> This header is based on ARM64 boot image header and provides an
> > >>> opportunity to combine both ARM64 & RISC-V image headers in future.
> > >>>
> > >>> Also make sure that PE/COFF header can co-exist in the same image so
> > >>> that EFI stub can be supported for RISC-V in future. EFI
> > >>> specification needs PE/COFF image header in the beginning of the
> > >>> kernel image in order to load it as an EFI application. In order to
> > >>> support EFI stub, code0 should be replaced with "MZ" magic string
> > >>> and res4(at offset 0x3c) should point to the rest of the PE/COFF
> > >>> header (which will be added during EFI support).
> > > [...]
> > >>> Documentation/riscv/boot-image-header.txt | 50
> > ++++++++++++++++++
> > >>> arch/riscv/include/asm/image.h            | 64 +++++++++++++++++++++++
> > >>> arch/riscv/kernel/head.S                  | 32 ++++++++++++
> > >>> 3 files changed, 146 insertions(+)
> > >>> create mode 100644 Documentation/riscv/boot-image-header.txt
> > >>> create mode 100644 arch/riscv/include/asm/image.h
> > >>>
> > >>> diff --git a/Documentation/riscv/boot-image-header.txt
> > >>> b/Documentation/riscv/boot-image-header.txt
> > >>> new file mode 100644
> > >>> index 000000000000..68abc2353cec
> > >>> --- /dev/null
> > >>> +++ b/Documentation/riscv/boot-image-header.txt
> > >>> @@ -0,0 +1,50 @@
> > >>> +                               Boot image header in RISC-V Linux
> > >>> +
> > >>> + =============================================
> > >>> +
> > >>> +Author: Atish Patra <atish.patra@wdc.com> Date  : 20 May 2019
> > >>> +
> > >>> +This document only describes the boot image header details for RISC-V
> > Linux.
> > >>> +The complete booting guide will be available at
> > Documentation/riscv/booting.txt.
> > >>> +
> > >>> +The following 64-byte header is present in decompressed Linux kernel
> > image.
> > >>> +
> > >>> +       u32 code0;                /* Executable code */
> > >>> +       u32 code1;                /* Executable code */
> > >>
> > >> Apologies for not mentioning this in my previous reply, but given
> > >> that you already know that you will need to put the magic string MZ
> > >> at offset 0x0, it makes more sense to not put any code there at all,
> > >> but educate the bootloader that the first executable instruction is
> > >> at offset 0x20, and put the spare fields right after it in case you
> > >> ever need more than 2 slots. (On arm64, we were lucky to be able to
> > >> find an opcode that happened to contain the MZ bit pattern and act
> > >> almost like a NOP, but it seems silly to rely on that for RISC-V as
> > >> well)
> > >>
> > >> So something like
> > >>
> > >> u16 pe_res1;  /* MZ for EFI bootable images, don't care otherwise */
> > >> u8 magic[6];    /* "RISCV\0"
> > >>
> > >> u64 text_offset;          /* Image load offset, little endian */
> > >> u64 image_size;           /* Effective Image size, little endian */
> > >> u64 flags;                /* kernel flags, little endian */
> > >>
> > >> u32 code0;                /* Executable code */
> > >> u32 code1;                /* Executable code */
> > >>
> > >> u64 reserved[2];     /* reserved for future use */
> > >>
> > >> u32 version;              /* Version of this header */
> > >> u32 pe_res2;                 /* Reserved for PE COFF offset */
> > >
> > > Hello,
> > >
> > > wouldn't that immediately break existing systems (including qemu when
> > > loading kernels with the "-kernel" option) that rely on the fact that
> > > the kernel entry point is always at the kernel load address?  The
> > > ARM64 header and Atish's original RISC-V proposal based on the ARM64
> > > header keep the property that jumping to the kernel load address
> > > always works, regardless of what the particular header looks like and
> > > which potential future extensions it includes, but the proposed change
> > > above wouldn't do that.
> > >
> > > Although I agree that having to integrate the "MZ" string as an
> > > instruction isn't particularly nice, I don't think that this is a
> > > sufficient justification for breaking compatibility with prior kernel
> > > releases and/or existing boot firmware.  On RISC-V, the "MZ" string is
> > > a compressed load immediate to x20/s4, i.e. an instruction that should
> > > be "harmless" as far as the kernel boot flow is concerned as the
> > > x20/s4 register AFAIK doesn't contain any information that the kernel
> > > would use.
> > >
> > > Regards,
> > > Karsten
> > >
> > 
> > Yes, that would break existing systems. Besides, the qemu -kernel option
> > uses the vmlinux elf file, and I think a better solution is make ‘loadelf’ work,
> > and include a second method for EFI.
> > 
> > (unfortunately, I had to drop some lists as I’m having trouble sending to
> > them via gmail, so the CC list on my response has been limited)
> 
> Nopes, it works perfectly fine on QEMU RISC-V.
> 
> Just like ARM64, we are lucky for RISC-V as well. The "MZ" string is a
> harmless load instruction in RISC-V so we don't need any changes in QEMU.

Hello,

just to avoid misunderstandings: Atish, does your "Nopes, it
works perfectly fine on QEMU RISC-V" refer to your original
header proposal or to Ard's modified header proposal?  For your
proposal I agree that it works without problems in all cases that
have worked before introduction of the header, i.e. adding your
proposed header is completely transparent, but as described above
I have doubts that the same is true for the (different) header
format that Ard has proposed above.

Regards,
Karsten
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