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LoongArch: Adjust symbol addressing for AS_HAS_EXPLICIT_RELOCS

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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>
This commit is contained in:
Xi Ruoyao 2022-10-12 16:36:08 +08:00 committed by Huacai Chen
parent 0d8dad7048
commit 11cd8a6483
4 changed files with 37 additions and 6 deletions
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18
arch/loongarch/Makefile
View File

@ -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)

9
arch/loongarch/include/asm/percpu.h
View File

@ -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");

12
arch/loongarch/kernel/head.S
View File

@ -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

4
arch/loongarch/kernel/vmlinux.lds.S
View File

@ -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 = .;