xen/riscv: introduce sbi call to putchar to console

Originally SBI implementation for Xen was introduced by
Bobby Eshleman <bobby.eshleman@gmail.com> but it was removed
all the stuff for simplicity  except SBI call for putting
character to console.

The patch introduces sbi_putchar() SBI call which is necessary
to implement initial early_printk.

Signed-off-by: Bobby Eshleman <bobby.eshleman@gmail.com>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Bobby Eshleman <bobby.eshleman@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

xen/arch/riscv/Makefile

@@ -1,4 +1,5 @@
 obj-$(CONFIG_RISCV_64) += riscv64/
+obj-y += sbi.o
 obj-y += setup.o
 
 $(TARGET): $(TARGET)-syms

xen/arch/riscv/include/asm/sbi.h

@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: (GPL-2.0-or-later) */
+/*
+ * Copyright (c) 2021-2023 Vates SAS.
+ *
+ * Taken from xvisor, modified by Bobby Eshleman (bobby.eshleman@gmail.com).
+ *
+ * Taken/modified from Xvisor project with the following copyright:
+ *
+ * Copyright (c) 2019 Western Digital Corporation or its affiliates.
+ */
+
+#ifndef __ASM_RISCV_SBI_H__
+#define __ASM_RISCV_SBI_H__
+
+#define SBI_EXT_0_1_CONSOLE_PUTCHAR		0x1
+
+struct sbiret {
+    long error;
+    long value;
+};
+
+struct sbiret sbi_ecall(unsigned long ext, unsigned long fid,
+                        unsigned long arg0, unsigned long arg1,
+                        unsigned long arg2, unsigned long arg3,
+                        unsigned long arg4, unsigned long arg5);
+
+/**
+ * Writes given character to the console device.
+ *
+ * @param ch The data to be written to the console.
+ */
+void sbi_console_putchar(int ch);
+
+#endif /* __ASM_RISCV_SBI_H__ */

xen/arch/riscv/sbi.c

@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Taken and modified from the xvisor project with the copyright Copyright (c)
+ * 2019 Western Digital Corporation or its affiliates and author Anup Patel
+ * (anup.patel@wdc.com).
+ *
+ * Modified by Bobby Eshleman (bobby.eshleman@gmail.com).
+ *
+ * Copyright (c) 2019 Western Digital Corporation or its affiliates.
+ * Copyright (c) 2021-2023 Vates SAS.
+ */
+
+#include <asm/sbi.h>
+
+struct sbiret sbi_ecall(unsigned long ext, unsigned long fid,
+                        unsigned long arg0, unsigned long arg1,
+                        unsigned long arg2, unsigned long arg3,
+                        unsigned long arg4, unsigned long arg5)
+{
+    struct sbiret ret;
+
+    register unsigned long a0 asm ("a0") = arg0;
+    register unsigned long a1 asm ("a1") = arg1;
+    register unsigned long a2 asm ("a2") = arg2;
+    register unsigned long a3 asm ("a3") = arg3;
+    register unsigned long a4 asm ("a4") = arg4;
+    register unsigned long a5 asm ("a5") = arg5;
+    register unsigned long a6 asm ("a6") = fid;
+    register unsigned long a7 asm ("a7") = ext;
+
+    asm volatile (  "ecall"
+                    : "+r" (a0), "+r" (a1)
+                    : "r" (a2), "r" (a3), "r" (a4), "r" (a5), "r" (a6), "r" (a7)
+                    : "memory");
+    ret.error = a0;
+    ret.value = a1;
+
+    return ret;
+}
+
+void sbi_console_putchar(int ch)
+{
+    sbi_ecall(SBI_EXT_0_1_CONSOLE_PUTCHAR, 0, ch, 0, 0, 0, 0, 0);
+}