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x86/Documentation: Update algo in init_size description of boot protocol

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BugLink: https://bugs.launchpad.net/bugs/2101915

[ Upstream commit be4ca6c53e66cb275cf0d71f32dac0c4606b9dc0 ]

The init_size description of boot protocol has an example of the runtime
start address for the compressed bzImage. For non-relocatable kernel
it relies on the pref_address value (if not 0), but for relocatable case
only pays respect to the load_addres and kernel_alignment, and it is
inaccurate for the latter. Boot loader must consider the pref_address
as the Linux kernel relocates to it before being decompressed as nicely
described in this commit message a year ago:

  43b1d3e68ee7 ("kexec: Allocate kernel above bzImage's pref_address")

Due to this documentation inaccuracy some of the bootloaders (*) made a
mistake in the calculations and if kernel image is big enough, this may
lead to unbootable configurations.

*)
  In particular, kexec-tools missed that and resently got a couple of
  changes which will be part of v2.0.30 release. For the record,
  commit 43b1d3e68ee7 only fixed the kernel kexec implementation and
  also missed to update the init_size description.

While at it, make an example C-like looking as it's done elsewhere in
the document and fix indentation as presribed by the reStructuredText
specifications, so the syntax highliting will work properly.

Fixes: 43b1d3e68ee7 ("kexec: Allocate kernel above bzImage's pref_address")
Fixes: d297366ba6 ("x86: document new bzImage fields")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20241125105005.1616154-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
This commit is contained in:
Andy Shevchenko
2025-03-11 08:51:19 +09:00
committed by Stefan Bader
parent e26d6e9c10
commit 8f6693142a
1 changed files with 13 additions and 4 deletions
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Documentation/arch/x86/boot.rst
+13 -4
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@@ -895,10 +895,19 @@ Offset/size: 0x260/4
The kernel runtime start address is determined by the following algorithm::
if (relocatable_kernel)
runtime_start = align_up(load_address, kernel_alignment)
else
runtime_start = pref_address
if (relocatable_kernel) {
if (load_address < pref_address)
load_address = pref_address;
runtime_start = align_up(load_address, kernel_alignment);
} else {
runtime_start = pref_address;
}
Hence the necessary memory window location and size can be estimated by
a boot loader as::
memory_window_start = runtime_start;
memory_window_size = init_size;
============ ===============
Field name: handover_offset