x86/sev: Don't emit BSS_DECRYPTED section unless it is in use

The BSS_DECRYPTED section that gets emitted into .bss will be empty if
CONFIG_AMD_MEM_ENCRYPT is not defined. However, due to the fact that it
is injected into .bss rather than emitted as a separate section, the
2 MiB alignment that it specifies is still taken into account
unconditionally, pushing .bss out to the next 2 MiB boundary, leaving
a gap that is never freed.

So only emit a non-empty BSS_DECRYPTED section if it is going to be
used.  In that case, it would still be nice to free the padding, but
that is left for later.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/20260108092526.28586-23-ardb@kernel.org

Author: ardbiesheuvel

arch/x86/kernel/vmlinux.lds.S

@@ -67,7 +67,18 @@ const_cpu_current_top_of_stack = cpu_current_top_of_stack;
 
 #define ALIGN_ENTRY_TEXT_BEGIN	. = ALIGN(PMD_SIZE);
 #define ALIGN_ENTRY_TEXT_END	. = ALIGN(PMD_SIZE);
+#else
+
+#define X86_ALIGN_RODATA_BEGIN
+#define X86_ALIGN_RODATA_END					\
+		. = ALIGN(PAGE_SIZE);				\
+		__end_rodata_aligned = .;
 
+#define ALIGN_ENTRY_TEXT_BEGIN
+#define ALIGN_ENTRY_TEXT_END
+#endif
+
+#ifdef CONFIG_AMD_MEM_ENCRYPT
 /*
  * This section contains data which will be mapped as decrypted. Memory
  * encryption operates on a page basis. Make this section PMD-aligned
@@ -88,17 +99,9 @@ const_cpu_current_top_of_stack = cpu_current_top_of_stack;
 	__pi___end_bss_decrypted = .;				\
 
 #else
-
-#define X86_ALIGN_RODATA_BEGIN
-#define X86_ALIGN_RODATA_END					\
-		. = ALIGN(PAGE_SIZE);				\
-		__end_rodata_aligned = .;
-
-#define ALIGN_ENTRY_TEXT_BEGIN
-#define ALIGN_ENTRY_TEXT_END
 #define BSS_DECRYPTED
-
 #endif
+
 #if defined(CONFIG_X86_64) && defined(CONFIG_KEXEC_CORE)
 #define KEXEC_RELOCATE_KERNEL					\
 	. = ALIGN(0x100);					\