From: Andy Shevchenko Date: Mon, 25 Nov 2024 10:49:14 +0000 (+0200) Subject: x86/Documentation: Update algo in init_size description of boot protocol X-Git-Url: http://git.hungrycats.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ac0ceec5c8f57a56c7bd342b5b45b505df775cca;p=linux x86/Documentation: Update algo in init_size description of boot protocol [ 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: d297366ba692 ("x86: document new bzImage fields") Signed-off-by: Andy Shevchenko Signed-off-by: Ingo Molnar Acked-by: Randy Dunlap Cc: "H. Peter Anvin" Link: https://lore.kernel.org/r/20241125105005.1616154-1-andriy.shevchenko@linux.intel.com Signed-off-by: Sasha Levin --- 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