On 11/29/22 21:03, Yanteng Si wrote:
On 11/29/22 10:40, WANG Xuerui wrote:
On 11/29/22 09:52, Yanteng Si wrote:
Describes the meaning and value of the image header field.
Suggested-by: Xiaotian Wu <wuxiaotian@xxxxxxxxxxx>
Signed-off-by: Yanteng Si <siyanteng@xxxxxxxxxxx>
---
Documentation/loongarch/booting.rst | 89
+++++++++++++++++++++++++++++
Documentation/loongarch/index.rst | 1 +
2 files changed, 90 insertions(+)
create mode 100644 Documentation/loongarch/booting.rst
diff --git a/Documentation/loongarch/booting.rst
b/Documentation/loongarch/booting.rst
new file mode 100644
index 000000000000..90456f713c48
--- /dev/null
+++ b/Documentation/loongarch/booting.rst
[snip]
+
+According to the EFI specification, the PE/COFF image file header is
+required at the beginning of the kernel image; the LoongArch kernel
+supports the EFI stub, so the first two bytes of the kernel image
header
+are "MZ" magic characters, and 0x3c should point to the rest of the
+PE/COFF file header, between which The kernel entry point, image
+effectivesize, and image load offset layout are as follows::
+
+ +-----------------------------------------------+
+ | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+ +-----------------------------------------------+
+ 0x00: | MZ_MAGIC | res0 |
+ +-----------------------------------------------+
+ 0x08: | kernel_entry |
+ +-----------------------------------------------+
+ 0x10: | _end - _text |
+ +-----------------------------------------------+
+ 0x18: | 0 |
+ +-----------------------------------------------+
+ 0x20: | res1 |
+ +-----------------------------------------------+
+ 0x28: | res2 |
+ +-----------------------------------------------+
+ 0x30: | res3 |
+ +-----------------------------------------------+
+ 0x38: | res4 | PE header |
+ +-----------------------------------------------+
This is repeating the structure described above, only in a pictorial
form. I think only one needs to be kept.
I don't think so.
The above corresponds to the struct in grub, this diagram is much more
readable.
How is an EFI stub image's header different in grub than somewhere else?
+
+All bit fields are in little endian:
+
+ - MZ: Effective length of 2 bytes;
+
+ - kernel entry point: leaving 6 bytes blank in front, starting
+ from 0x8, with a effective length of 8 bytes;
+
+ - kernel image effective size: Immediately following the kernel
+ entry point, with a effective length of 8 bytes, this field is
+ required for the bootloader;
The boot loader probably doesn't care. GRUB2 just chain-loads after
recognizing the image as valid EFI application, and the firmware most
certainly just do its routine EFI application launch too without
looking at the contents here that are all in fact just reserved
space. (IIUC only the MZ signature and the PE header offset matter
here; everything in between is actually part of the DOS stub which is
ignored on all modern platforms.)
So, please clarify which component requires this info; is it the EFI
stub instead? (I haven't checked myself though due to limited review
time. Sorry for that.)
I wrote this article with partial reference to riscv.
See Documentation/riscv/boot-image-header.rst line 61:
- Image size is mandatory for boot loader to load kernel image.
Booting will
fail otherwise.
No, the LoongArch EFI boot protocol is not the same as that of RISC-V,
we've taken lessons and I believe the LoongArch GRUB2 simply chain-loads
the image without any of the extra Linux-specific checking. Please read
the previous EFI boot patch threads carefully, and you can also ask
Xiaotian or Huacai for details (they're both CC'd on this thread so they
should have the context).
+
+ - kernel image load offset: Immediately following the kernel
+ image effective size, with a effective length of 8 bytes;
+
+ - Offset to the PE header: leaving 28 bytes blank in front,
+ starting from 0x3c, with a effective length of 4 bytes.
Drop explanation for trivial things like MZ or the PE header offset,
they're just common PE format thing, and totally unrelated to Linux.
Here's just a simple description of the header bit fields, which is a
prerequisite for understanding the boot image header.
Since the boot image is just a PE file, and it's clearly mandated in the
UEFI spec, I think some knowledge of the PE format should be necessary
after all? In which case the reader could be pointed to the PE spec for
the parts not related to Linux or LoongArch, and this document can stay
focused on its main topic.
Thanks,
Yanteng
diff --git a/Documentation/loongarch/index.rst
b/Documentation/loongarch/index.rst
index aaba648db907..df0174d6d227 100644
--- a/Documentation/loongarch/index.rst
+++ b/Documentation/loongarch/index.rst
@@ -10,6 +10,7 @@ LoongArch Architecture
introduction
irq-chip-model
+ booting
features
--
WANG "xen0n" Xuerui
Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/
