riscv: support early_printk

Because printk() relies on a serial driver (like the ns16550 driver)
and drivers require working virtual memory (ioremap()) there is not
print functionality early in Xen boot.

This commit adds early printk implementation built on the putc SBI call.

Signed-off-by: Bobby Eshleman <bobby.eshleman@gmail.com>

xen/arch/riscv/Makefile

@@ -4,6 +4,7 @@ obj-y   += domctl.o
 obj-y   += domain.o
 obj-y   += domain_build.o
 obj-y   += delay.o
+obj-y   += early_printk.o
 obj-y   += entry.o
 obj-y   += guestcopy.o
 obj-y   += irq.o

xen/arch/riscv/early_printk.c

@@ -0,0 +1,48 @@
+/*
+ * RISC-V early printk using SBI
+ *
+ * Copyright (C) 2021 Bobby Eshleman <bobbyeshleman@gmail.com>
+ */
+#include <asm/sbi.h>
+#include <asm/early_printk.h>
+#include <xen/stdarg.h>
+#include <xen/lib.h>
+
+void _early_puts(const char *s, size_t nr)
+{
+    while ( nr-- > 0 )
+    {
+        if (*s == '\n')
+            sbi_console_putchar('\r');
+        sbi_console_putchar(*s);
+        s++;
+    }
+}
+
+static void vprintk_early(const char *prefix, const char *fmt, va_list args)
+{
+    char buf[128];
+    int sz;
+
+    early_puts(prefix);
+
+    sz = vscnprintf(buf, sizeof(buf), fmt, args);
+
+    if ( sz < 0 ) {
+        early_puts("(XEN) vprintk_early error\n");
+        return;
+    }
+
+    if ( sz == 0 )
+        return;
+
+    _early_puts(buf, sz);
+}
+
+void early_printk(const char *fmt, ...)
+{
+    va_list args;
+    va_start(args, fmt);
+    vprintk_early("(XEN) ", fmt, args);
+    va_end(args);
+}

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

@@ -0,0 +1,10 @@
+#ifndef __EARLY_PRINTK_H__
+#define __EARLY_PRINTK_H__
+
+#include <xen/string.h>
+
+#define early_puts(s) _early_puts((s), strlen((s)))
+void _early_puts(const char *s, size_t nr);
+void early_printk(const char *fmt, ...);
+
+#endif /* __EARLY_PRINTK_H__ */