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

[Randy Dunlap <rdunlap@infradead.org>]

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@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?

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