arch/xtensa: Add call0-based syscall interface

There was a system call handler added as part of the call0 ABI
integration.  Wire it up to the standard Zephyr userspace system call
API.

Signed-off-by: Andy Ross <andyross@google.com>

Author: andyross

arch/xtensa/core/xtensa-win0.S

@@ -474,3 +474,47 @@ xtensa_switch:
 	s32i a4, a3, 0 /* store switch handle */
 	mov a3, a2
 	j restore
+
+/* This file also contains the system call stubs, though those are
+ * used only by ring 3 code and live in the regular .text section.
+ * The system calls work just like CALL0 ABI functions, with arguments
+ * marshalled for us by the compiler.  All we need to do is put the ID
+ * number in A11.
+ */
+.section .text.asm_syscall_stubs, "ax"
+
+.global xtensa_syscall6
+xtensa_syscall6:
+	mov a11, a8
+	syscall
+	ret
+
+.global xtensa_syscall5
+xtensa_syscall5:
+	mov a11, a7
+	syscall
+	ret
+
+.global xtensa_syscall4
+xtensa_syscall4:
+	mov a11, a6
+	syscall
+	ret
+
+.global xtensa_syscall3
+xtensa_syscall3:
+	mov a11, a5
+	syscall
+	ret
+
+.global xtensa_syscall2
+xtensa_syscall2:
+	mov a11, a4
+	syscall
+	ret
+
+.global xtensa_syscall1
+xtensa_syscall1:
+	mov a11, a3
+	syscall
+	ret

include/zephyr/arch/syscall.h

@@ -23,6 +23,8 @@
 #include <zephyr/arch/arc/syscall.h>
 #elif defined(CONFIG_RISCV)
 #include <zephyr/arch/riscv/syscall.h>
+#elif defined(CONFIG_XTENSA)
+#include <zephyr/arch/xtensa/syscall.h>
 #endif
 
 #endif /* ZEPHYR_INCLUDE_ARCH_SYSCALL_H_ */

include/zephyr/arch/xtensa/syscall.h

@@ -0,0 +1,37 @@
+/* Copyright 2023 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#ifndef ZEPHYR_INCLUDE_ARCH_XTENSA_SYSCALL_H_
+#define ZEPHYR_INCLUDE_ARCH_XTENSA_SYSCALL_H_
+
+/* The Xtensa syscall interface is a straightforward extension of the
+ * CALL0 function ABI convention, the only difference being placement
+ * of the system call ID (the C declarations for Zephyr put it in the
+ * last argument, which is variable, where the handler wants to see it
+ * in A11 always.  Therefore the system calls are expressed as tiny
+ * linkable functions in xtensa-call0.S.
+ *
+ * Alas the declarations for the Zephyr API have these functions
+ * tagged inline, so we need to do a little preprocessor work to avoid
+ * redeclaration warnings.
+ */
+
+uintptr_t xtensa_syscall6(uintptr_t a1, uintptr_t a2, uintptr_t a3,
+			  uintptr_t a4, uintptr_t a5, uintptr_t a6, uintptr_t id);
+uintptr_t xtensa_syscall5(uintptr_t a1, uintptr_t a2, uintptr_t a3,
+			  uintptr_t a4, uintptr_t a5, uintptr_t id);
+uintptr_t xtensa_syscall4(uintptr_t a1, uintptr_t a2, uintptr_t a3,
+			  uintptr_t a4, uintptr_t id);
+uintptr_t xtensa_syscall3(uintptr_t a1, uintptr_t a2, uintptr_t a3,
+			  uintptr_t id);
+uintptr_t xtensa_syscall2(uintptr_t a1, uintptr_t a2, uintptr_t id);
+uintptr_t xtensa_syscall1(uintptr_t a1, uintptr_t id);
+
+#define arch_syscall_invoke6 xtensa_syscall6
+#define arch_syscall_invoke5 xtensa_syscall5
+#define arch_syscall_invoke4 xtensa_syscall4
+#define arch_syscall_invoke3 xtensa_syscall3
+#define arch_syscall_invoke2 xtensa_syscall2
+#define arch_syscall_invoke1 xtensa_syscall1
+
+#endif /* ZEPHYR_INCLUDE_ARCH_XTENSA_SYSCALL_H_ */