[PATCH 3/4] RISC-V: export the ISA string of the running machine in the aux vector

[Heiko Stuebner]

From: Heiko Stuebner <heiko.stuebner@xxxxxxxx>

Already defined aux-vectors regarding the machine type are AT_PLATFORM
and AT_BASE_PLATFORM. PPC already uses AT_BASE_PLATFORM to identify the
real platform the system is running on, so do a similar thing on RISC-V
and export the ISA string of the running machine via this aux-vector
element.

This way userspace can possibly adapt to extensions that allow it to
run certain loads more performantly.

Signed-off-by: Heiko Stuebner <heiko.stuebner@xxxxxxxx>
---
 arch/riscv/include/asm/elf.h | 10 ++++++++++
 arch/riscv/kernel/cpu.c      | 12 ++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/riscv/include/asm/elf.h b/arch/riscv/include/asm/elf.h
index 30e7d2455960..820ef627e83d 100644
--- a/arch/riscv/include/asm/elf.h
+++ b/arch/riscv/include/asm/elf.h
@@ -78,6 +78,16 @@ extern unsigned long elf_hwcap;
 
 #define COMPAT_ELF_PLATFORM	(NULL)
 
+/*
+ * ELF_PLATFORM indicates the ISA supported by the platform, but has
+ * special meaning to ld.so .
+ * Expose the ISA string including all usable extensions via
+ * ELF_BASE_PLATFORM instead and allow userspace to adapt to them
+ * if needed.
+ */
+#define ELF_BASE_PLATFORM (riscv_base_platform)
+extern const char *riscv_base_platform;
+
 #ifdef CONFIG_MMU
 #define ARCH_DLINFO						\
 do {								\
diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index 06c2f587a176..71770563199f 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -118,8 +118,12 @@ static int riscv_cpuinfo_starting(unsigned int cpu)
 	return 0;
 }
 
+const char *riscv_base_platform = NULL;
+static char *riscv_create_isa_string(void);
+
 static int __init riscv_cpuinfo_init(void)
 {
+	char *isa_str;
 	int ret;
 
 	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "riscv/cpuinfo:starting",
@@ -129,6 +133,14 @@ static int __init riscv_cpuinfo_init(void)
 		return ret;
 	}
 
+	/*
+	 * Create the isa-string with the common set of extensions over
+	 * all harts, to expose as AT_BASE_PLATFORM in the aux vector.
+	 */
+	isa_str = riscv_create_isa_string();
+	if (!IS_ERR(isa_str))
+		riscv_base_platform = isa_str;
+
 	return 0;
 }
 arch_initcall(riscv_cpuinfo_init);
-- 
2.39.0