linux-kernel - [PATCH v6 3/6] LoongArch: Use model("extreme") attribute for per-CPU variables in module if CONFIG_AS_HAS_EXPLICIT

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Message-Id: <20220829133146.15236-4-xry111@xry111.site>
Date:   Mon, 29 Aug 2022 21:31:43 +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 v6 3/6] LoongArch: Use model("extreme") attribute for per-CPU variables in module if CONFIG_AS_HAS_EXPLICIT_RELOCS

On LoongArch, The "address" of a per-CPU variable 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.  We need to tell the
compiler that a PC-relative addressing with 32-bit offset is not
sufficient for local per-CPU variables.

After some discussion with GCC maintainers, we implemented this
attribute to indicate that a PC-relative addressing with 64-bit offset
should be used.

This attribute should be available in GCC 13 release.  Some early GCC 13
snapshots may support -mexplicit-relocs but lack this attribute, we
simply reject them.

Signed-off-by: Xi Ruoyao <xry111@...111.site>
Link: https://gcc.gnu.org/r13-2199
---
 arch/loongarch/Makefile             | 3 +++
 arch/loongarch/include/asm/percpu.h | 8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index 1563747c4fa8..593818a61741 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -53,6 +53,9 @@ LDFLAGS_vmlinux			+= -G0 -static -n -nostdlib
 # 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
+ifeq ($(shell echo '__has_attribute(model)' | $(CC) -E -P - 2> /dev/null), 0)
+$(error "C compiler must support model attribute if using explicit relocs")
+endif
 cflags-y			+= -mexplicit-relocs
 else
 cflags-y			+= $(call cc-option,-mno-explicit-relocs)
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");
 
-- 
2.37.0