riscv: Support XUANTIE extended CSR save/restore during task switching

arch/riscv/Kconfig

@@ -721,6 +721,18 @@ config XUANTIE_ISA
 
 	  If unsure, say N.
 
+config XUANTIE_CSR_EXT
+	bool "XUANTIE_CSR_EXT (FXCR, UTNMODE)"
+	default y
+	depends on FPU
+	help
+	  This config enable XUANTIE U-mode Extended Control and Status Register.
+	  These CSRs may be set to different values ​​by the user in different processes, and
+	  can be saved and restored when switching tasks in combination with dts.
+	  Please refer to the XUANTIE processor user manual for CSR definitions.
+
+	  If you don't know what to do here, say Y.
+
 menu "Kernel features"
 
 source "kernel/Kconfig.hz"

arch/riscv/include/asm/csr.h

@@ -423,6 +423,12 @@
 #define CSR_XRLENB		0xcc1
 #define CSR_XMISA		0xcc2
 
+/* XUANTIE Extended Control and Status Register */
+#ifdef CONFIG_XUANTIE_CSR_EXT
+#define CSR_FXCR		0x800
+#define CSR_UTNMODE		0x8da
+#endif
+
 #ifdef CONFIG_RISCV_M_MODE
 # define CSR_STATUS	CSR_MSTATUS
 # define CSR_IE		CSR_MIE

arch/riscv/include/asm/hwcap.h

@@ -60,6 +60,8 @@
 #define RISCV_ISA_EXT_ZIHPM		42
 #define RISCV_ISA_EXT_ZAWRS		85
 
+#define RISCV_ISA_EXT_XTHEADFXCR	123
+#define RISCV_ISA_EXT_XTHEADUTNMODE	124
 #define RISCV_ISA_EXT_XTHEADVECTOR      125
 #define RISCV_ISA_EXT_XTHEADMATRIX	126
 

arch/riscv/include/asm/processor.h

@@ -83,6 +83,10 @@ struct thread_struct {
 	unsigned long vstate_ctrl;
 	struct __riscv_v_ext_state vstate;
 	struct __riscv_m_ext_state mstate;
+#ifdef CONFIG_XUANTIE_CSR_EXT
+	u32 fxcr;
+	u32 utnmode;
+#endif
 } __attribute__((__aligned__(sizeof(xlen_t))));
 
 /* Whitelist the fstate from the task_struct for hardened usercopy */

arch/riscv/include/asm/switch_to.h

@@ -13,6 +13,7 @@
 #include <asm/processor.h>
 #include <asm/ptrace.h>
 #include <asm/csr.h>
+#include <asm/xuantie_csr_ext.h>
 
 #ifdef CONFIG_FPU
 extern void __fstate_save(struct task_struct *save_to);
@@ -83,6 +84,8 @@ do {							\
 		__switch_to_vector(__prev, __next);	\
 	if (has_matrix())				\
 		__switch_to_matrix(__prev, __next);	\
+	if (has_xuantie_csr_ext())				\
+		__switch_to_xuantie_csr_ext(__prev, __next);	\
 	((last) = __switch_to(__prev, __next));		\
 } while (0)
 

arch/riscv/include/asm/xuantie_csr_ext.h

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_XUANTIE_CSR_EXT_H
+#define _ASM_XUANTIE_CSR_EXT_H
+
+#ifdef CONFIG_XUANTIE_CSR_EXT
+
+#include <asm/csr.h>
+#include <asm/hwcap.h>
+#include <linux/sched.h>
+
+static __always_inline bool has_xuantie_csr_ext(void)
+{
+	return riscv_has_extension_unlikely(RISCV_ISA_EXT_XTHEADFXCR) |
+		riscv_has_extension_unlikely(RISCV_ISA_EXT_XTHEADUTNMODE);
+}
+
+static inline void __switch_to_xuantie_csr_ext(struct task_struct *prev,
+				      struct task_struct *next)
+{
+	if (riscv_has_extension_likely(RISCV_ISA_EXT_XTHEADFXCR)) {
+		csr_set(CSR_STATUS, SR_FS);
+		prev->thread.fxcr = csr_read(CSR_FXCR);
+		csr_write(CSR_FXCR, next->thread.fxcr);
+		csr_clear(CSR_STATUS, SR_FS);
+	}
+
+	if (riscv_has_extension_likely(RISCV_ISA_EXT_XTHEADUTNMODE)) {
+		prev->thread.utnmode = csr_read(CSR_UTNMODE);
+		csr_write(CSR_UTNMODE, next->thread.utnmode);
+	}
+}
+#else
+
+static __always_inline bool has_xuantie_csr_ext(void) { return false; }
+#define __switch_to_xuantie_csr_ext(__prev, __next)	do {} while (0)
+
+#endif /* CONFIG_XUANTIE_CSR_EXT */
+#endif /* _ASM_XUANTIE_CSR_EXT_H */

arch/riscv/kernel/cpufeature.c

@@ -185,6 +185,8 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
 	__RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
 	__RISCV_ISA_EXT_DATA(xtheadmatrix, RISCV_ISA_EXT_XTHEADMATRIX),
 	__RISCV_ISA_EXT_DATA(xtheadvector, RISCV_ISA_EXT_XTHEADVECTOR),
+	__RISCV_ISA_EXT_DATA(xtheadfxcr, RISCV_ISA_EXT_XTHEADFXCR),
+	__RISCV_ISA_EXT_DATA(xtheadutnmode, RISCV_ISA_EXT_XTHEADUTNMODE),
 };
 
 const size_t riscv_isa_ext_count = ARRAY_SIZE(riscv_isa_ext);