Re: [Xen-devel] [PATCH v3 1/3] x86/boot: Introduce the kernel_info

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

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?

> > +  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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