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Date: Tue, 30 Aug 2022 18:48:03 +0800
From: Xi Ruoyao <xry111@...111.site>
To: loongarch@...ts.linux.dev
Cc: linux-kernel@...r.kernel.org, WANG Xuerui <kernel@...0n.name>,
Huacai Chen <chenhuacai@...nel.org>,
Youling Tang <tangyouling@...ngson.cn>,
Jinyang He <hejinyang@...ngson.cn>,
Xi Ruoyao <xry111@...111.site>
Subject: [PATCH v7 2/5] LoongArch: Adjust symbol addressing for CONFIG_AS_HAS_EXPLICIT_RELOCS
If explicit relocation hints is used by the toolchain, -Wa,-mla-*
options will be useless for C code. Only use them for
!CONFIG_AS_HAS_EXPLICIT_RELOCS.
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 nearing the loading address of main
kernel image, but far from the address of modules. Use model("extreme")
attibute to tell the compiler that a 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 reloc explicit reloc * Behavior
==============================================================
No No Use la.* macros.
No change from Linux 6.0.
--------------------------------------------------------------
No Yes Disable explicit reloc.
No change from Linux 6.0.
--------------------------------------------------------------
Yes No Not supported.
--------------------------------------------------------------
Yes Yes Use explicit relocs.
No -Wa,-mla* options.
==============================================================
*: We assume CC must have model attribute if it has explicit reloc.
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
Signed-off-by: Xi Ruoyao <xry111@...111.site>
---
arch/loongarch/Makefile | 17 +++++++++++++++++
arch/loongarch/include/asm/percpu.h | 8 ++++++++
arch/loongarch/kernel/head.S | 10 +++++-----
3 files changed, 30 insertions(+), 5 deletions(-)
diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index 7051a95f7f31..92c4a52c4c3e 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -40,10 +40,27 @@ 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
+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 0bd6b0110198..dd7fcc553efa 100644
--- a/arch/loongarch/include/asm/percpu.h
+++ b/arch/loongarch/include/asm/percpu.h
@@ -8,6 +8,14 @@
#include <asm/cmpxchg.h>
#include <asm/loongarch.h>
+#if defined(MODULE) && defined(CONFIG_AS_HAS_EXPLICIT_RELOCS)
+/* The "address" (in fact, offset from $r21) of a per-CPU variable is close
+ * to the load address of main kernel image, but far from where the modules are
+ * loaded. Tell the compiler this fact.
+ */
+# define PER_CPU_ATTRIBUTES __attribute__((model("extreme")))
+#endif
+
/* Use r21 for fast access */
register unsigned long __my_cpu_offset __asm__("$r21");
diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
index 01bac62a6442..eb3f641d5915 100644
--- a/arch/loongarch/kernel/head.S
+++ b/arch/loongarch/kernel/head.S
@@ -55,17 +55,17 @@ 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
/* KSave3 used for percpu base, initialized as 0 */
@@ -73,7 +73,7 @@ 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
--
2.37.0
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