On 10/10/19 2:43 AM, Daniel Kiper wrote: > On Wed, Oct 09, 2019 at 05:43:31PM -0700, Randy Dunlap wrote: >> Hi, >> >> Questions and comments below... >> Thanks. >> >> On 10/9/19 3:53 AM, Daniel Kiper wrote: >> >>> Suggested-by: H. Peter Anvin <hpa@xxxxxxxxx> >>> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx> >>> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> >>> Reviewed-by: Ross Philipson <ross.philipson@xxxxxxxxxx> >>> --- >> >>> --- >>> 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? > > s/It/The kernel_info structure/ Is it better? Yes. >>> + 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/ > > OK. > >>> +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? > > EsTT == Example STrucT > EsTs == Example STringS > > Anyway, it can be anything which does not collide with existing variable > length data magics. There are none right now. So, it can be anything. > Maybe I should add something saying that. Yes, please. thanks. -- ~Randy
