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SubjectRe: [PATCH v3 1/3] x86/boot: Introduce the kernel_info
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Hi,

Questions and comments below...
Thanks.


On 10/9/19 3:53 AM, Daniel Kiper wrote:

> Suggested-by: H. Peter Anvin <hpa@zytor.com>
> Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Reviewed-by: Ross Philipson <ross.philipson@oracle.com>
> ---

> ---
> Documentation/x86/boot.rst | 121 +++++++++++++++++++++++++++++++++
> arch/x86/boot/Makefile | 2 +-
> arch/x86/boot/compressed/Makefile | 4 +-
> arch/x86/boot/compressed/kernel_info.S | 17 +++++
> arch/x86/boot/header.S | 1 +
> arch/x86/boot/tools/build.c | 5 ++
> arch/x86/include/uapi/asm/bootparam.h | 1 +
> 7 files changed, 148 insertions(+), 3 deletions(-)
> create mode 100644 arch/x86/boot/compressed/kernel_info.S
>
> diff --git a/Documentation/x86/boot.rst b/Documentation/x86/boot.rst
> index 08a2f100c0e6..d5323a39f5e3 100644
> --- a/Documentation/x86/boot.rst
> +++ b/Documentation/x86/boot.rst
> @@ -68,8 +68,25 @@ Protocol 2.12 (Kernel 3.8) Added the xloadflags field and extension fields
> Protocol 2.13 (Kernel 3.14) Support 32- and 64-bit flags being set in
> xloadflags to support booting a 64-bit kernel from 32-bit
> EFI
> +
> +Protocol 2.14: BURNT BY INCORRECT COMMIT ae7e1238e68f2a472a125673ab506d49158c1889
> + (x86/boot: Add ACPI RSDP address to setup_header)
> + DO NOT USE!!! ASSUME SAME AS 2.13.
> +
> +Protocol 2.15: (Kernel 5.5) Added the kernel_info.
> ============= ============================================================
>
> +.. note::
> + The protocol version number should be changed only if the setup header
> + is changed. There is no need to update the version number if boot_params
> + or kernel_info are changed. Additionally, it is recommended to use
> + xloadflags (in this case the protocol version number should not be
> + updated either) or kernel_info to communicate supported Linux kernel
> + features to the boot loader. Due to very limited space available in
> + the original setup header every update to it should be considered
> + with great care. Starting from the protocol 2.15 the primary way to
> + communicate things to the boot loader is the kernel_info.
> +
>
> Memory Layout
> =============
> @@ -207,6 +224,7 @@ Offset/Size Proto Name Meaning
> 0258/8 2.10+ pref_address Preferred loading address
> 0260/4 2.10+ init_size Linear memory required during initialization
> 0264/4 2.11+ handover_offset Offset of handover entry point
> +0268/4 2.15+ kernel_info_offset Offset of the kernel_info
> =========== ======== ===================== ============================================
>
> .. note::
> @@ -855,6 +873,109 @@ Offset/size: 0x264/4
>
> See EFI HANDOVER PROTOCOL below for more details.
>
> +============ ==================
> +Field name: kernel_info_offset
> +Type: read
> +Offset/size: 0x268/4
> +Protocol: 2.15+
> +============ ==================
> +
> + This field is the offset from the beginning of the kernel image to the
> + kernel_info. It is embedded in the Linux image in the uncompressed
^^
What does It refer to, please?

> + protected mode region.
> +
> +
> +The kernel_info
> +===============
> +
> +The relationships between the headers are analogous to the various data
> +sections:
> +
> + setup_header = .data
> + boot_params/setup_data = .bss
> +
> +What is missing from the above list? That's right:
> +
> + kernel_info = .rodata
> +
> +We have been (ab)using .data for things that could go into .rodata or .bss for
> +a long time, for lack of alternatives and -- especially early on -- inertia.
> +Also, the BIOS stub is responsible for creating boot_params, so it isn't
> +available to a BIOS-based loader (setup_data is, though).
> +
> +setup_header is permanently limited to 144 bytes due to the reach of the
> +2-byte jump field, which doubles as a length field for the structure, combined
> +with the size of the "hole" in struct boot_params that a protected-mode loader
> +or the BIOS stub has to copy it into. It is currently 119 bytes long, which
> +leaves us with 25 very precious bytes. This isn't something that can be fixed
> +without revising the boot protocol entirely, breaking backwards compatibility.
> +
> +boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
> +by adding setup_data entries. It cannot be used to communicate properties of
> +the kernel image, because it is .bss and has no image-provided content.
> +
> +kernel_info solves this by providing an extensible place for information about
> +the kernel image. It is readonly, because the kernel cannot rely on a
> +bootloader copying its contents anywhere, but that is OK; if it becomes
> +necessary it can still contain data items that an enabled bootloader would be
> +expected to copy into a setup_data chunk.
> +
> +All kernel_info data should be part of this structure. Fixed size data have to
> +be put before kernel_info_var_len_data label. Variable size data have to be put
> +behind kernel_info_var_len_data label. Each chunk of variable size data has to

s/behind/after/

> +be prefixed with header/magic and its size, e.g.:
> +
> + kernel_info:
> + .ascii "LToP" /* Header, Linux top (structure). */
> + .long kernel_info_var_len_data - kernel_info
> + .long kernel_info_end - kernel_info
> + .long 0x01234567 /* Some fixed size data for the bootloaders. */
> + kernel_info_var_len_data:
> + example_struct: /* Some variable size data for the bootloaders. */
> + .ascii "EsTT" /* Header/Magic. */
> + .long example_struct_end - example_struct
> + .ascii "Struct"
> + .long 0x89012345
> + example_struct_end:
> + example_strings: /* Some variable size data for the bootloaders. */
> + .ascii "EsTs" /* Header/Magic. */

Where do the Magic values "EsTT" and "EsTs" come from?
where are they defined?

> + .long example_strings_end - example_strings
> + .asciz "String_0"
> + .asciz "String_1"
> + example_strings_end:
> + kernel_info_end:
> +
> +This way the kernel_info is self-contained blob.
> +
> +
> +Details of the kernel_info Fields
> +=================================
> +
> +============ ========
> +Field name: header
> +Offset/size: 0x0000/4
> +============ ========
> +
> + Contains the magic number "LToP" (0x506f544c).
> +
> +============ ========
> +Field name: size
> +Offset/size: 0x0004/4
> +============ ========
> +
> + This field contains the size of the kernel_info including kernel_info.header.
> + It does not count kernel_info.kernel_info_var_len_data size. This field should be
> + used by the bootloaders to detect supported fixed size fields in the kernel_info
> + and beginning of kernel_info.kernel_info_var_len_data.
> +
> +============ ========
> +Field name: size_total
> +Offset/size: 0x0008/4
> +============ ========
> +
> + This field contains the size of the kernel_info including kernel_info.header
> + and kernel_info.kernel_info_var_len_data.
> +
>
> The Image Checksum
> ==================


--
~Randy

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