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

[Ard Biesheuvel]

On Tue, 28 May 2019 at 12:34, Anup Patel <Anup.Patel@xxxxxxx> wrote:
>
>
>
> > -----Original Message-----
> > From: Karsten Merker <merker@xxxxxxxxxx>
> > Sent: Tuesday, May 28, 2019 1:53 PM
> > To: Anup Patel <Anup.Patel@xxxxxxx>
> > Cc: Troy Benjegerdes <troy.benjegerdes@xxxxxxxxxx>; Karsten Merker
> > <merker@xxxxxxxxxx>; Albert Ou <aou@xxxxxxxxxxxxxxxxx>; Jonathan
> > Corbet <corbet@xxxxxxx>; Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>;
> > linux-kernel@xxxxxxxxxxxxxxx List <linux-kernel@xxxxxxxxxxxxxxx>; Zong Li
> > <zong@xxxxxxxxxxxxx>; Atish Patra <Atish.Patra@xxxxxxx>; Palmer
> > Dabbelt <palmer@xxxxxxxxxx>; paul.walmsley@xxxxxxxxxx; Nick Kossifidis
> > <mick@xxxxxxxxxxxx>; linux-riscv@xxxxxxxxxxxxxxxxxxx;
> > marek.vasut@xxxxxxxxx
> > Subject: Re: [v4 PATCH] RISC-V: Add an Image header that boot loader can
> > parse.
> >
> > On Tue, May 28, 2019 at 03:54:02AM +0000, Anup Patel wrote:
> > > > From: Troy Benjegerdes <troy.benjegerdes@xxxxxxxxxx>
> > > > > On May 27, 2019, at 5:16 PM, Karsten Merker <merker@xxxxxxxxxx>
> > > > 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@xxxxxxx>
> > 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@xxxxxxx> 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 âloadelfâ work, and include a second method for EFI.
> > > >
> > > > (unfortunately, I had to drop some lists as Iâm 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
>
> Sorry for the confusion. I meant here that adding "MZ" at start in Atish's
> proposed header works fine on QEMU.
>
> > 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.
>
> Yes, Ard's proposed header will break booting on current QEMU and
> existing HW. I think Ard's proposed header was to address the case if
> "MZ" was not a valid and harmless instruction in RISC-V. Other than
> that Ard's proposal is similar to Atish's proposal but organized differently.
>
> For Atish's proposed header, we are certainly relying on the fact that
> "MZ" represents a valid and harmless instruction (just like ARM64) but
> this approach is allowing us to boot Linux RISC-V kernel without breaking
> existing booting methods.
>

Fair enough. If you guys can live with it, then so can I :-)