x86/mm: Identify the end of the kernel area to be reserved

The memory occupied by the kernel is reserved using memblock_reserve()
in setup_arch(). Currently, the area is from symbols _text to __bss_stop.
Everything after __bss_stop must be specifically reserved otherwise it
is discarded. This is not clearly documented.

Add a new symbol, __end_of_kernel_reserve, that more readily identifies
what is reserved, along with comments that indicate what is reserved,
what is discarded and what needs to be done to prevent a section from
being discarded.

Signed-off-by: Tom Lendacky <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Reviewed-by: Baoquan He <[e-mail]>
Reviewed-by: Dave Hansen <[e-mail]>
Tested-by: Lianbo Jiang <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Brijesh Singh <[e-mail]>
Cc: Dave Young <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Joerg Roedel <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Nick Desaulniers <[e-mail]>
Cc: Pavel Tatashin <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Robert Richter <[e-mail]>
Cc: Sami Tolvanen <[e-mail]>
Cc: Sinan Kaya <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: "[e-mail]" <[e-mail]>
Link: https://lkml.kernel.org/r/7db7da4[e-mail]

arch/x86/include/asm/sections.h

···
 extern char __end_rodata_hpage_align[];
 #endif
 
+extern char __end_of_kernel_reserve[];
+
 #endif	/* _ASM_X86_SECTIONS_H */

arch/x86/kernel/setup.c

···
 
 void __init setup_arch(char **cmdline_p)
 {
+	/*
+	 * Reserve the memory occupied by the kernel between _text and
+	 * __end_of_kernel_reserve symbols. Any kernel sections after the
+	 * __end_of_kernel_reserve symbol must be explicitly reserved with a
+	 * separate memblock_reserve() or they will be discarded.
+	 */
 	memblock_reserve(__pa_symbol(_text),
-			 (unsigned long)__bss_stop - (unsigned long)_text);
+			 (unsigned long)__end_of_kernel_reserve - (unsigned long)_text);
 
 	/*
 	 * Make sure page 0 is always reserved because on systems with

arch/x86/kernel/vmlinux.lds.S

···
 		__bss_stop = .;
 	}
 
+	/*
+	 * The memory occupied from _text to here, __end_of_kernel_reserve, is
+	 * automatically reserved in setup_arch(). Anything after here must be
+	 * explicitly reserved using memblock_reserve() or it will be discarded
+	 * and treated as available memory.
+	 */
+	__end_of_kernel_reserve = .;
+
 	. = ALIGN(PAGE_SIZE);
 	.brk : AT(ADDR(.brk) - LOAD_OFFSET) {
 		__brk_base = .;
···
 	STABS_DEBUG
 	DWARF_DEBUG
 
-	/* Sections to be discarded */
 	DISCARDS
 	/DISCARD/ : {
 		*(.eh_frame)