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

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: d297366ba692 ("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

diff --git a/Documentation/arch/x86/boot.rst b/Documentation/arch/x86/boot.rst
index 4fd492cb49704fa21fd368fe0832f332e952c83d..ad2d8ddad27fe43119f8f7f389687317e5da6571 100644 (file)
--- a/Documentation/arch/x86/boot.rst
+++ b/Documentation/arch/x86/boot.rst
@@ -896,10 +896,19 @@ 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