LoongArch: Add support for relocating the kernel with RELR relocation

RELR as a relocation packing format for relative relocations for
reducing the size of relative relocation records.  In a position
independent executable there are often many relative relocation
records, and our vmlinux is a PIE.

The LLD linker (since 17.0.0) and the BFD linker (since 2.43) supports
packing the relocations in the RELR format for LoongArch, with the flag
-z pack-relative-relocs.

Commits 5cf896fb ("arm64: Add support for relocating the kernel
with RELR relocations") and ccb2d173 ("Makefile: use -z
pack-relative-relocs") have already added the framework to use RELR.
We just need to wire it up and process the RELR relocation records in
relocate_relative() in addition to the RELA relocation records.

A ".p2align 3" directive is added to la_abs macro or the BFD linker
cannot pack the relocation records against the .la_abs section (the
". = ALIGN(8);" directive in vmlinux.lds.S is too late in the linking
process).

With defconfig and CONFIG_RELR vmlinux.efi is 2.1 MiB (6%) smaller, and
vmlinuz.efi (using gzip compression) is 384 KiB (2.8%) smaller.

Link: https://groups.google.com/d/topic/generic-abi/bX460iggiKg
Link: https://reviews.llvm.org/D138135#4531389
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d89ecf33ab6dSigned-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -610,6 +610,7 @@ config ARCH_HAS_GENERIC_CRASHKERNEL_RESERVATION
 
 config RELOCATABLE
 	bool "Relocatable kernel"
+	select ARCH_HAS_RELR
 	help
 	  This builds the kernel as a Position Independent Executable (PIE),
 	  which retains all relocation metadata required, so as to relocate

arch/loongarch/include/asm/asmmacro.h

@@ -609,6 +609,7 @@
 	lu32i.d	\reg, 0
 	lu52i.d	\reg, \reg, 0
 	.pushsection ".la_abs", "aw", %progbits
+	.p2align 3
 	.dword	766b
 	.dword	\sym
 	.popsection

arch/loongarch/include/asm/setup.h

@@ -34,6 +34,11 @@ extern long __la_abs_end;
 extern long __rela_dyn_begin;
 extern long __rela_dyn_end;
 
+#ifdef CONFIG_RELR
+extern long __relr_dyn_begin;
+extern long __relr_dyn_end;
+#endif
+
 extern unsigned long __init relocate_kernel(void);
 
 #endif

arch/loongarch/kernel/relocate.c

@@ -38,6 +38,24 @@ static inline void __init relocate_relative(void)
 		relocated_addr = (Elf64_Addr)RELOCATED(relocated_addr);
 		*(Elf64_Addr *)RELOCATED(addr) = relocated_addr;
 	}
+
+#ifdef CONFIG_RELR
+	u64 *addr = NULL;
+	u64 *relr = (u64 *)&__relr_dyn_begin;
+	u64 *relr_end = (u64 *)&__relr_dyn_end;
+
+	for ( ; relr < relr_end; relr++) {
+		if ((*relr & 1) == 0) {
+			addr = (u64 *)(*relr + reloc_offset);
+			*addr++ += reloc_offset;
+		} else {
+			for (u64 *p = addr, r = *relr >> 1; r; p++, r >>= 1)
+				if (r & 1)
+					*p += reloc_offset;
+			addr += 63;
+		}
+	}
+#endif
 }
 
 static inline void __init relocate_absolute(long random_offset)

arch/loongarch/kernel/vmlinux.lds.S

@@ -113,6 +113,14 @@ SECTIONS
 		__rela_dyn_end = .;
 	}
 
+#ifdef CONFIG_RELR
+	.relr.dyn : ALIGN(8) {
+		__relr_dyn_begin = .;
+		 *(.relr.dyn)
+		__relr_dyn_end = .;
+	}
+#endif
+
 	.data.rel : { *(.data.rel*) }
 
 #ifdef CONFIG_RELOCATABLE