Re: [PATCH v3 1/3] x86/boot: Introduce the kernel_info
From: Randy Dunlap
Date: Thu Oct 10 2019 - 10:43:58 EST
On 10/10/19 2:43 AM, Daniel Kiper wrote:
> On Wed, Oct 09, 2019 at 05:43:31PM -0700, Randy Dunlap wrote:
>> Questions and comments below...
>> 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
>>> +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?
>>> + protected mode region.
>>> +The kernel_info
>>> +The relationships between the headers are analogous to the various data
>>> + 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
>>> +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.