riscv: hwcap: change ELF_HWCAP to a function

Using a function is flexible to represent ELF_HWCAP. So the kernel may
encode hwcap reflecting supported hardware features just at the moment of
the start of each program.

This will be helpful when we introduce prctl/sysctl interface to control
per-process availability of Vector extension in following patches.
Programs started with V disabled should see V masked off in theirs
ELF_HWCAP.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230605110724.21391-21-andy.chiu@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/elf.h

@@ -66,7 +66,7 @@ extern bool compat_elf_check_arch(Elf32_Ehdr *hdr);
  * via a bitmap that coorespends to each single-letter ISA extension.  This is
  * essentially defunct, but will remain for compatibility with userspace.
  */
-#define ELF_HWCAP	(elf_hwcap & ((1UL << RISCV_ISA_EXT_BASE) - 1))
+#define ELF_HWCAP	riscv_get_elf_hwcap()
 extern unsigned long elf_hwcap;
 
 /*

arch/riscv/include/asm/hwcap.h

@@ -61,6 +61,8 @@
 
 #include <linux/jump_label.h>
 
+unsigned long riscv_get_elf_hwcap(void);
+
 struct riscv_isa_ext_data {
 	/* Name of the extension displayed to userspace via /proc/cpuinfo */
 	char uprop[RISCV_ISA_EXT_NAME_LEN_MAX];

arch/riscv/kernel/cpufeature.c

@@ -293,6 +293,11 @@ void __init riscv_fill_hwcap(void)
 	pr_info("riscv: ELF capabilities %s\n", print_str);
 }
 
+unsigned long riscv_get_elf_hwcap(void)
+{
+	return (elf_hwcap & ((1UL << RISCV_ISA_EXT_BASE) - 1));
+}
+
 #ifdef CONFIG_RISCV_ALTERNATIVE
 /*
  * Alternative patch sites consider 48 bits when determining when to patch