LoongArch: Adjust symbol addressing for AS_HAS_EXPLICIT_RELOCS

If explicit relocation hints are used by the toolchain, -Wa,-mla-*
options will be useless for the C code. So only use them for the
!CONFIG_AS_HAS_EXPLICIT_RELOCS case.

Replace "la" with "la.pcrel" in head.S to keep the semantic consistent
with new and old toolchains for the low level startup code.

For per-CPU variables, the "address" of the symbol is actually an offset
from $r21. The value is near the loading address of main kernel image,
but far from the loading address of modules. So we use model("extreme")
attibute to tell the compiler that a PC-relative addressing with 32-bit
offset is not sufficient for local per-CPU variables.

The behavior with different assemblers and compilers are summarized in
the following table:

AS has            CC has
explicit relocs   explicit relocs * Behavior
==============================================================
No                No                Use la.* macros.
                                    No change from Linux 6.0.
--------------------------------------------------------------
No                Yes               Disable explicit relocs.
                                    No change from Linux 6.0.
--------------------------------------------------------------
Yes               No                Not supported.
--------------------------------------------------------------
Yes               Yes               Enable explicit relocs.
                                    No -Wa,-mla* options used.
==============================================================
*: We assume CC must have model attribute if it has explicit relocs.
   Both features are added in GCC 13 development cycle, so any GCC
   release >= 13 should be OK. Using early GCC 13 development snapshots
   may produce modules with unsupported relocations.

Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=f09482a
Link: https://gcc.gnu.org/r13-1834
Link: https://gcc.gnu.org/r13-2199
Tested-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index 84689c3..42352f9 100644 (file)
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -43,10 +43,28 @@ endif
 
 cflags-y                       += -G0 -pipe -msoft-float
 LDFLAGS_vmlinux                        += -G0 -static -n -nostdlib
+
+# When the assembler supports explicit relocation hint, we must use it.
+# GCC may have -mexplicit-relocs off by default if it was built with an old
+# assembler, so we force it via an option.
+#
+# When the assembler does not supports explicit relocation hint, we can't use
+# it.  Disable it if the compiler supports it.
+#
+# If you've seen "unknown reloc hint" message building the kernel and you are
+# now wondering why "-mexplicit-relocs" is not wrapped with cc-option: the
+# combination of a "new" assembler and "old" compiler is not supported.  Either
+# upgrade the compiler or downgrade the assembler.
+ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS
+cflags-y                       += -mexplicit-relocs
+KBUILD_CFLAGS_KERNEL           += -mdirect-extern-access
+else
+cflags-y                       += $(call cc-option,-mno-explicit-relocs)
 KBUILD_AFLAGS_KERNEL           += -Wa,-mla-global-with-pcrel
 KBUILD_CFLAGS_KERNEL           += -Wa,-mla-global-with-pcrel
 KBUILD_AFLAGS_MODULE           += -Wa,-mla-global-with-abs
 KBUILD_CFLAGS_MODULE           += -fplt -Wa,-mla-global-with-abs,-mla-local-with-abs
+endif
 
 cflags-y += -ffreestanding
 cflags-y += $(call cc-option, -mno-check-zero-division)

diff --git a/arch/loongarch/include/asm/percpu.h b/arch/loongarch/include/asm/percpu.h
index 0bd6b01..ad8d884 100644 (file)
--- a/arch/loongarch/include/asm/percpu.h
+++ b/arch/loongarch/include/asm/percpu.h
@@ -8,6 +8,15 @@
 #include <asm/cmpxchg.h>
 #include <asm/loongarch.h>
 
+/*
+ * The "address" (in fact, offset from $r21) of a per-CPU variable is close to
+ * the loading address of main kernel image, but far from where the modules are
+ * loaded. Tell the compiler this fact when using explicit relocs.
+ */
+#if defined(MODULE) && defined(CONFIG_AS_HAS_EXPLICIT_RELOCS)
+#define PER_CPU_ATTRIBUTES    __attribute__((model("extreme")))
+#endif
+
 /* Use r21 for fast access */
 register unsigned long __my_cpu_offset __asm__("$r21");
 

diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
index 7e57ae8..0c67c24 100644 (file)
--- a/arch/loongarch/kernel/head.S
+++ b/arch/loongarch/kernel/head.S
@@ -57,19 +57,19 @@ SYM_CODE_START(kernel_entry)                        # kernel entry point
        li.w            t0, 0x00                # FPE=0, SXE=0, ASXE=0, BTE=0
        csrwr           t0, LOONGARCH_CSR_EUEN
 
-       la              t0, __bss_start         # clear .bss
+       la.pcrel        t0, __bss_start         # clear .bss
        st.d            zero, t0, 0
-       la              t1, __bss_stop - LONGSIZE
+       la.pcrel        t1, __bss_stop - LONGSIZE
 1:
        addi.d          t0, t0, LONGSIZE
        st.d            zero, t0, 0
        bne             t0, t1, 1b
 
-       la              t0, fw_arg0
+       la.pcrel        t0, fw_arg0
        st.d            a0, t0, 0               # firmware arguments
-       la              t0, fw_arg1
+       la.pcrel        t0, fw_arg1
        st.d            a1, t0, 0
-       la              t0, fw_arg2
+       la.pcrel        t0, fw_arg2
        st.d            a2, t0, 0
 
        /* KSave3 used for percpu base, initialized as 0 */
@@ -77,7 +77,7 @@ SYM_CODE_START(kernel_entry)                  # kernel entry point
        /* GPR21 used for percpu base (runtime), initialized as 0 */
        move            u0, zero
 
-       la              tp, init_thread_union
+       la.pcrel        tp, init_thread_union
        /* Set the SP after an empty pt_regs.  */
        PTR_LI          sp, (_THREAD_SIZE - 32 - PT_SIZE)
        PTR_ADD         sp, sp, tp

diff --git a/arch/loongarch/kernel/vmlinux.lds.S b/arch/loongarch/kernel/vmlinux.lds.S
index e5890be..b3309a5 100644 (file)
--- a/arch/loongarch/kernel/vmlinux.lds.S
+++ b/arch/loongarch/kernel/vmlinux.lds.S
@@ -55,6 +55,10 @@ SECTIONS
 
        EXCEPTION_TABLE(16)
 
+       .got : ALIGN(16) { *(.got) }
+       .plt : ALIGN(16) { *(.plt) }
+       .got.plt : ALIGN(16) { *(.got.plt) }
+
        . = ALIGN(PECOFF_SEGMENT_ALIGN);
        __init_begin = .;
        __inittext_begin = .;