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SubjectRe: [PATCH v3 1/3] x86/boot: Introduce the kernel_info
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@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?

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
> > +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.

Daniel

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