linux-kernel - [PATCH v2 1/1] x86/Documentation: Update algo in init

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Message-ID: <20241125105005.1616154-1-andriy.shevchenko@linux.intel.com>
Date: Mon, 25 Nov 2024 12:49:14 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Jonathan Corbet <corbet@....net>,
	Cloud Hsu <cloudhsu@...gle.com>,
	Chris Koch <chrisko@...gle.com>
Subject: [PATCH v2 1/1] 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 the commit 43b1d3e68ee7 message.

Due to this 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,
  the 43b1d3e68ee7 fixed only 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@...ux.intel.com>
---

v2: fixed the style to follow both rST and kernel conventions (Ingo, Randy)

 Documentation/arch/x86/boot.rst | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/Documentation/arch/x86/boot.rst b/Documentation/arch/x86/boot.rst
index 4fd492cb4970..ad2d8ddad27f 100644
--- 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
-- 
2.43.0.rc1.1336.g36b5255a03ac