riscv: Move global pointer loading to a macro

In Clang 17, -fsanitize=shadow-call-stack uses the newly declared
platform register gp for storing shadow call stack pointers. As
this is obviously incompatible with gp relaxation, in preparation
for CONFIG_SHADOW_CALL_STACK support, move global pointer loading
to a single macro, which we can cleanly disable when SCS is used
instead.

Link: https://reviews.llvm.org/rGaa1d2693c256
Link: riscv-non-isa/riscv-elf-psabi-doc@a484e84
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20230927224757.1154247-11-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: samitolvanen

arch/riscv/include/asm/asm.h

@@ -109,6 +109,14 @@
 	REG_L \dst, 0(\dst)
 .endm
 
+/* load __global_pointer to gp */
+.macro load_global_pointer
+.option push
+.option norelax
+	la gp, __global_pointer$
+.option pop
+.endm
+
 	/* save all GPs except x1 ~ x5 */
 	.macro save_from_x6_to_x31
 	REG_S x6,  PT_T1(sp)

arch/riscv/kernel/entry.S

@@ -75,10 +75,8 @@ _save_context:
 	csrw CSR_SCRATCH, x0
 
 	/* Load the global pointer */
-.option push
-.option norelax
-	la gp, __global_pointer$
-.option pop
+	load_global_pointer
+
 	move a0, sp /* pt_regs */
 	la ra, ret_from_exception
 

arch/riscv/kernel/head.S

@@ -110,10 +110,7 @@ relocate_enable_mmu:
 	csrw CSR_TVEC, a0
 
 	/* Reload the global pointer */
-.option push
-.option norelax
-	la gp, __global_pointer$
-.option pop
+	load_global_pointer
 
 	/*
 	 * Switch to kernel page tables.  A full fence is necessary in order to
@@ -134,10 +131,7 @@ secondary_start_sbi:
 	csrw CSR_IP, zero
 
 	/* Load the global pointer */
-	.option push
-	.option norelax
-		la gp, __global_pointer$
-	.option pop
+	load_global_pointer
 
 	/*
 	 * Disable FPU & VECTOR to detect illegal usage of
@@ -228,10 +222,7 @@ pmp_done:
 #endif /* CONFIG_RISCV_M_MODE */
 
 	/* Load the global pointer */
-.option push
-.option norelax
-	la gp, __global_pointer$
-.option pop
+	load_global_pointer
 
 	/*
 	 * Disable FPU & VECTOR to detect illegal usage of

arch/riscv/kernel/suspend_entry.S

@@ -61,10 +61,7 @@ END(__cpu_suspend_enter)
 
 SYM_TYPED_FUNC_START(__cpu_resume_enter)
 	/* Load the global pointer */
-	.option push
-	.option norelax
-		la gp, __global_pointer$
-	.option pop
+	load_global_pointer
 
 #ifdef CONFIG_MMU
 	/* Save A0 and A1 */