lib: sbi: Introduce sbi_trap_exit() API

We introduce sbi_trap_exit() API which can help non-firmware
(i.e. generic or platform) code to force exit trap/interrupt
handling and resume execution at context pointed by parameter
"const struct sbi_trap_regs *regs".

This new sbi_trap_exit() API will help Keystone Enclave project
to resume execution of enclave from custom SBI call handler.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>

firmware/fw_base.S

@@ -261,6 +261,9 @@ _scratch_init:
 	/* Store hartid-to-scratch function address in scratch space */
 	la	a4, _hartid_to_scratch
 	REG_S	a4, SBI_SCRATCH_HARTID_TO_SCRATCH_OFFSET(tp)
+	/* Store trap-exit function address in scratch space */
+	la	a4, _trap_exit
+	REG_S	a4, SBI_SCRATCH_TRAP_EXIT_OFFSET(tp)
 	/* Clear tmp0 in scratch space */
 	REG_S	zero, SBI_SCRATCH_TMP0_OFFSET(tp)
 	/* Store firmware options in scratch space */
@@ -423,6 +426,18 @@ _skip_trap_handler_rv32_hyp:
 #endif
 	csrw	CSR_MTVEC, a4
 
+#if __riscv_xlen == 32
+	/* Override trap exit for H-extension */
+	csrr	a5, CSR_MISA
+	srli	a5, a5, ('H' - 'A')
+	andi	a5, a5, 0x1
+	beq	a5, zero, _skip_trap_exit_rv32_hyp
+	la	a4, _trap_exit_rv32_hyp
+	csrr	a5, CSR_MSCRATCH
+	REG_S	a4, SBI_SCRATCH_TRAP_EXIT_OFFSET(a5)
+_skip_trap_exit_rv32_hyp:
+#endif
+
 	/* Initialize SBI runtime */
 	csrr	a0, CSR_MSCRATCH
 	call	sbi_init
@@ -653,6 +668,20 @@ _trap_handler:
 
 	mret
 
+	.section .entry, "ax", %progbits
+	.align 3
+	.globl _trap_exit
+_trap_exit:
+	add	sp, a0, zero
+
+	TRAP_RESTORE_GENERAL_REGS_EXCEPT_SP_T0
+
+	TRAP_RESTORE_MEPC_MSTATUS 0
+
+	TRAP_RESTORE_SP_T0
+
+	mret
+
 #if __riscv_xlen == 32
 	.section .entry, "ax", %progbits
 	.align 3
@@ -673,6 +702,20 @@ _trap_handler_rv32_hyp:
 	TRAP_RESTORE_SP_T0
 
 	mret
+
+	.section .entry, "ax", %progbits
+	.align 3
+	.globl _trap_exit_rv32_hyp
+_trap_exit_rv32_hyp:
+	add	sp, a0, zero
+
+	TRAP_RESTORE_GENERAL_REGS_EXCEPT_SP_T0
+
+	TRAP_RESTORE_MEPC_MSTATUS 1
+
+	TRAP_RESTORE_SP_T0
+
+	mret
 #endif
 
 	.section .entry, "ax", %progbits

include/sbi/sbi_scratch.h

@@ -30,12 +30,14 @@
 #define SBI_SCRATCH_PLATFORM_ADDR_OFFSET	(6 * __SIZEOF_POINTER__)
 /** Offset of hartid_to_scratch member in sbi_scratch */
 #define SBI_SCRATCH_HARTID_TO_SCRATCH_OFFSET	(7 * __SIZEOF_POINTER__)
+/** Offset of trap_exit member in sbi_scratch */
+#define SBI_SCRATCH_TRAP_EXIT_OFFSET		(8 * __SIZEOF_POINTER__)
 /** Offset of tmp0 member in sbi_scratch */
-#define SBI_SCRATCH_TMP0_OFFSET			(8 * __SIZEOF_POINTER__)
+#define SBI_SCRATCH_TMP0_OFFSET			(9 * __SIZEOF_POINTER__)
 /** Offset of options member in sbi_scratch */
-#define SBI_SCRATCH_OPTIONS_OFFSET		(9 * __SIZEOF_POINTER__)
+#define SBI_SCRATCH_OPTIONS_OFFSET		(10 * __SIZEOF_POINTER__)
 /** Offset of extra space in sbi_scratch */
-#define SBI_SCRATCH_EXTRA_SPACE_OFFSET		(10 * __SIZEOF_POINTER__)
+#define SBI_SCRATCH_EXTRA_SPACE_OFFSET		(11 * __SIZEOF_POINTER__)
 /** Maximum size of sbi_scratch (4KB) */
 #define SBI_SCRATCH_SIZE			(0x1000)
 
@@ -63,6 +65,8 @@ struct sbi_scratch {
 	unsigned long platform_addr;
 	/** Address of HART ID to sbi_scratch conversion function */
 	unsigned long hartid_to_scratch;
+	/** Address of trap exit function */
+	unsigned long trap_exit;
 	/** Temporary storage */
 	unsigned long tmp0;
 	/** Options for OpenSBI library */

include/sbi/sbi_trap.h

@@ -207,6 +207,8 @@ int sbi_trap_redirect(struct sbi_trap_regs *regs,
 
 void sbi_trap_handler(struct sbi_trap_regs *regs);
 
+void __noreturn sbi_trap_exit(const struct sbi_trap_regs *regs);
+
 #endif
 
 #endif

lib/sbi/sbi_trap.c

@@ -16,6 +16,7 @@
 #include <sbi/sbi_illegal_insn.h>
 #include <sbi/sbi_ipi.h>
 #include <sbi/sbi_misaligned_ldst.h>
+#include <sbi/sbi_scratch.h>
 #include <sbi/sbi_timer.h>
 #include <sbi/sbi_trap.h>
 
@@ -271,3 +272,22 @@ trap_error:
 	if (rc)
 		sbi_trap_error(msg, rc, mcause, mtval, mtval2, mtinst, regs);
 }
+
+typedef void (*trap_exit_t)(const struct sbi_trap_regs *regs);
+
+/**
+ * Exit trap/interrupt handling
+ *
+ * This function is called by non-firmware code to abruptly exit
+ * trap/interrupt handling and resume execution at context pointed
+ * by given register state.
+ *
+ * @param regs pointer to register state
+ */
+void __noreturn sbi_trap_exit(const struct sbi_trap_regs *regs)
+{
+	struct sbi_scratch *scratch = sbi_scratch_thishart_ptr();
+
+	((trap_exit_t)scratch->trap_exit)(regs);
+	__builtin_unreachable();
+}