riscv: add Smcsrind indirect CSR access extension support

arch/riscv/Kconfig.isa

@@ -117,6 +117,15 @@ config RISCV_ISA_EXT_ZICSR
 	  The "Zicsr" extension introduces support for the full set of CSR
 	  instructions that operate on CSRs registers.
 
+config RISCV_ISA_EXT_SMCSRIND
+	bool
+	select RISCV_ISA_EXT_ZICSR
+	help
+	  (Smcsrind) - Standard Extension for Indirect CSR Access
+
+	  The Smcsrind extension provides indirect access to CSRs through
+	  the MISELECT and MIREG registers.
+
 config RISCV_ISA_EXT_ZIFENCEI
 	bool
 	help

include/zephyr/arch/riscv/csr.h

@@ -1,4 +1,4 @@
 /*
  * Copyright (c) 2020 Michael Schaffner
  * Copyright (c) 2020 BayLibre, SAS
  *
@@ -233,4 +233,44 @@
 				: "memory");			\
 })
 
+#ifdef CONFIG_RISCV_ISA_EXT_SMCSRIND
+
+#define MISELECT 0x350
+#define MIREG    0x351
+#define MIREG2   0x352
+#define MIREG3   0x353
+#define MIREG4   0x355
+#define MIREG5   0x356
+#define MIREG6   0x357
+
+static inline unsigned long icsr_read(unsigned int index)
+{
+	csr_write(MISELECT, index);
+	return csr_read(MIREG);
+}
+
+static inline void icsr_write(unsigned int index, unsigned long value)
+{
+	csr_write(MISELECT, index);
+	csr_write(MIREG, value);
+}
+
+static inline unsigned long icsr_read_set(unsigned int index, unsigned long mask)
+{
+	unsigned long val = icsr_read(index);
+
+	icsr_write(index, val | mask);
+	return val;
+}
+
+static inline unsigned long icsr_read_clear(unsigned int index, unsigned long mask)
+{
+	unsigned long val = icsr_read(index);
+
+	icsr_write(index, val & ~mask);
+	return val;
+}
+
+#endif /* CONFIG_RISCV_ISA_EXT_SMCSRIND */
+
 #endif /* CSR_H_ */