semihosting: implement a semihosting console

This provides two functions for handling console output that handle
the common backend behaviour for semihosting.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

hw/semihosting/Makefile.objs

@@ -1 +1,2 @@
 obj-$(CONFIG_SEMIHOSTING) += config.o
+obj-$(CONFIG_SEMIHOSTING) += console.o

hw/semihosting/console.c

@@ -0,0 +1,77 @@
+/*
+ * Semihosting Console Support
+ *
+ * Copyright (c) 2015 Imagination Technologies
+ * Copyright (c) 2019 Linaro Ltd
+ *
+ * This provides support for outputting to a semihosting console.
+ *
+ * While most semihosting implementations support reading and writing
+ * to arbitrary file descriptors we treat the console as something
+ * specifically for debugging interaction. This means messages can be
+ * re-directed to gdb (if currently being used to debug) or even
+ * re-directed elsewhere.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "cpu.h"
+#include "hw/semihosting/console.h"
+#include "exec/gdbstub.h"
+#include "qemu/log.h"
+
+int qemu_semihosting_log_out(const char *s, int len)
+{
+    return write(STDERR_FILENO, s, len);
+}
+
+/*
+ * A re-implementation of lock_user_string that we can use locally
+ * instead of relying on softmmu-semi. Hopefully we can deprecate that
+ * in time. We either copy len bytes if specified or until we find a NULL.
+ */
+static GString *copy_user_string(CPUArchState *env, target_ulong addr, int len)
+{
+    CPUState *cpu = ENV_GET_CPU(env);
+    GString *s = g_string_sized_new(len ? len : 128);
+    uint8_t c;
+    bool done;
+
+    do {
+        if (cpu_memory_rw_debug(cpu, addr++, &c, 1, 0) == 0) {
+            s = g_string_append_c(s, c);
+            done = len ? s->len == len : c == 0;
+        } else {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                          "%s: passed inaccessible address " TARGET_FMT_lx,
+                          __func__, addr);
+            done = true;
+        }
+    } while (!done);
+
+    return s;
+}
+
+static void semihosting_cb(CPUState *cs, target_ulong ret, target_ulong err)
+{
+    if (ret == (target_ulong) -1) {
+        qemu_log("%s: gdb console output failed ("TARGET_FMT_ld")",
+                 __func__, err);
+    }
+}
+
+int qemu_semihosting_console_out(CPUArchState *env, target_ulong addr, int len)
+{
+    GString *s = copy_user_string(env, addr, len);
+    int out = s->len;
+
+    if (use_gdb_syscalls()) {
+        gdb_do_syscall(semihosting_cb, "write,2,%x,%x", addr, s->len);
+    } else {
+        out = qemu_semihosting_log_out(s->str, s->len);
+    }
+
+    g_string_free(s, true);
+    return out;
+}

include/hw/semihosting/console.h

@@ -0,0 +1,38 @@
+/*
+ * Semihosting Console
+ *
+ * Copyright (c) 2019 Linaro Ltd
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#ifndef _SEMIHOST_CONSOLE_H_
+#define _SEMIHOST_CONSOLE_H_
+
+/**
+ * qemu_semihosting_console_out:
+ * @env: CPUArchState
+ * @s: host address of guest string
+ * @len: length of string or 0 (string is null terminated)
+ *
+ * Send a guest string to the debug console. This may be the remote
+ * gdb session if a softmmu guest is currently being debugged.
+ *
+ * Returns: number of bytes written.
+ */
+int qemu_semihosting_console_out(CPUArchState *env, target_ulong s, int len);
+
+/**
+ * qemu_semihosting_log_out:
+ * @s: pointer to string
+ * @len: length of string
+ *
+ * Send a string to the debug output. Unlike console_out these strings
+ * can't be sent to a remote gdb instance as they don't exist in guest
+ * memory.
+ *
+ * Returns: number of bytes written
+ */
+int qemu_semihosting_log_out(const char *s, int len);
+
+#endif /* _SEMIHOST_CONSOLE_H_ */