semihosting: add qemu_semihosting_console_inc for SYS_READC

Provides a blocking call to read a character from the console using
semihosting.chardev, if specified. This takes some careful command
line options to use stdio successfully as the serial ports, monitor
and semihost all want to use stdio. Here's a sample set of command
line options which share stdio between semihost, monitor and serial
ports:

	qemu \
	-chardev stdio,mux=on,id=stdio0 \
	-serial chardev:stdio0 \
	-semihosting-config enable=on,chardev=stdio0 \
	-mon chardev=stdio0,mode=readline

This creates a chardev hooked to stdio and then connects all of the
subsystems to it. A shorter mechanism would be good to hear about.

Signed-off-by: Keith Packard <keithp@keithp.com>
Message-Id: <20191104204230.12249-1-keithp@keithp.com>
[AJB: fixed up deadlock, minor commit title reword]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Tested-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

hw/semihosting/console.c

@@ -20,8 +20,15 @@
 #include "hw/semihosting/semihost.h"
 #include "hw/semihosting/console.h"
 #include "exec/gdbstub.h"
+#include "exec/exec-all.h"
 #include "qemu/log.h"
 #include "chardev/char.h"
+#include <pthread.h>
+#include "chardev/char-fe.h"
+#include "sysemu/sysemu.h"
+#include "qemu/main-loop.h"
+#include "qapi/error.h"
+#include "qemu/fifo8.h"
 
 int qemu_semihosting_log_out(const char *s, int len)
 {
@@ -98,3 +105,75 @@ void qemu_semihosting_console_outc(CPUArchState *env, target_ulong addr)
                       __func__, addr);
     }
 }
+
+#define FIFO_SIZE   1024
+
+/* Access to this structure is protected by the BQL */
+typedef struct SemihostingConsole {
+    CharBackend         backend;
+    GSList              *sleeping_cpus;
+    bool                got;
+    Fifo8               fifo;
+} SemihostingConsole;
+
+static SemihostingConsole console;
+
+static int console_can_read(void *opaque)
+{
+    SemihostingConsole *c = opaque;
+    int ret;
+    g_assert(qemu_mutex_iothread_locked());
+    ret = (int) fifo8_num_free(&c->fifo);
+    return ret;
+}
+
+static void console_wake_up(gpointer data, gpointer user_data)
+{
+    CPUState *cs = (CPUState *) data;
+    /* cpu_handle_halt won't know we have work so just unbung here */
+    cs->halted = 0;
+    qemu_cpu_kick(cs);
+}
+
+static void console_read(void *opaque, const uint8_t *buf, int size)
+{
+    SemihostingConsole *c = opaque;
+    g_assert(qemu_mutex_iothread_locked());
+    while (size-- && !fifo8_is_full(&c->fifo)) {
+        fifo8_push(&c->fifo, *buf++);
+    }
+    g_slist_foreach(c->sleeping_cpus, console_wake_up, NULL);
+    c->sleeping_cpus = NULL;
+}
+
+target_ulong qemu_semihosting_console_inc(CPUArchState *env)
+{
+    uint8_t ch;
+    SemihostingConsole *c = &console;
+    g_assert(qemu_mutex_iothread_locked());
+    g_assert(current_cpu);
+    if (fifo8_is_empty(&c->fifo)) {
+        c->sleeping_cpus = g_slist_prepend(c->sleeping_cpus, current_cpu);
+        current_cpu->halted = 1;
+        current_cpu->exception_index = EXCP_HALTED;
+        cpu_loop_exit(current_cpu);
+        /* never returns */
+    }
+    ch = fifo8_pop(&c->fifo);
+    return (target_ulong) ch;
+}
+
+void qemu_semihosting_console_init(void)
+{
+    Chardev *chr = semihosting_get_chardev();
+
+    if  (chr) {
+        fifo8_create(&console.fifo, FIFO_SIZE);
+        qemu_chr_fe_init(&console.backend, chr, &error_abort);
+        qemu_chr_fe_set_handlers(&console.backend,
+                                 console_can_read,
+                                 console_read,
+                                 NULL, NULL, &console,
+                                 NULL, true);
+    }
+}

include/hw/semihosting/console.h

@@ -37,6 +37,22 @@ int qemu_semihosting_console_outs(CPUArchState *env, target_ulong s);
  */
 void qemu_semihosting_console_outc(CPUArchState *env, target_ulong c);
 
+/**
+ * qemu_semihosting_console_inc:
+ * @env: CPUArchState
+ *
+ * Receive single character from debug console. This may be the remote
+ * gdb session if a softmmu guest is currently being debugged. As this
+ * call may block if no data is available we suspend the CPU and will
+ * re-execute the instruction when data is there. Therefore two
+ * conditions must be met:
+ *   - CPUState is synchronized before calling this function
+ *   - pc is only updated once the character is successfully returned
+ *
+ * Returns: character read OR cpu_loop_exit!
+ */
+target_ulong qemu_semihosting_console_inc(CPUArchState *env);
+
 /**
  * qemu_semihosting_log_out:
  * @s: pointer to string

include/hw/semihosting/semihost.h

@@ -56,6 +56,9 @@ static inline Chardev *semihosting_get_chardev(void)
 {
     return NULL;
 }
+static inline void qemu_semihosting_console_init(void)
+{
+}
 #else /* !CONFIG_USER_ONLY */
 bool semihosting_enabled(void);
 SemihostingTarget semihosting_get_target(void);
@@ -68,6 +71,7 @@ Chardev *semihosting_get_chardev(void);
 void qemu_semihosting_enable(void);
 int qemu_semihosting_config_options(const char *opt);
 void qemu_semihosting_connect_chardevs(void);
+void qemu_semihosting_console_init(void);
 #endif /* CONFIG_USER_ONLY */
 
 #endif /* SEMIHOST_H */

linux-user/arm/semihost.c

@@ -14,6 +14,7 @@
 #include "cpu.h"
 #include "hw/semihosting/console.h"
 #include "qemu.h"
+#include <termios.h>
 
 int qemu_semihosting_console_outs(CPUArchState *env, target_ulong addr)
 {
@@ -47,3 +48,29 @@ void qemu_semihosting_console_outc(CPUArchState *env, target_ulong addr)
         }
     }
 }
+
+/*
+ * For linux-user we can safely block. However as we want to return as
+ * soon as a character is read we need to tweak the termio to disable
+ * line buffering. We restore the old mode afterwards in case the
+ * program is expecting more normal behaviour. This is slow but
+ * nothing using semihosting console reading is expecting to be fast.
+ */
+target_ulong qemu_semihosting_console_inc(CPUArchState *env)
+{
+    uint8_t c;
+    struct termios old_tio, new_tio;
+
+    /* Disable line-buffering and echo */
+    tcgetattr(STDIN_FILENO, &old_tio);
+    new_tio = old_tio;
+    new_tio.c_lflag &= (~ICANON & ~ECHO);
+    tcsetattr(STDIN_FILENO, TCSANOW, &new_tio);
+
+    c = getchar();
+
+    /* restore config */
+    tcsetattr(STDIN_FILENO, TCSANOW, &old_tio);
+
+    return (target_ulong) c;
+}

stubs/semihost.c

@@ -69,3 +69,7 @@ void semihosting_arg_fallback(const char *file, const char *cmd)
 void qemu_semihosting_connect_chardevs(void)
 {
 }
+
+void qemu_semihosting_console_init(void)
+{
+}

target/arm/arm-semi.c

@@ -802,8 +802,7 @@ target_ulong do_arm_semihosting(CPUARMState *env)
 
         return guestfd_fns[gf->type].readfn(cpu, gf, arg1, len);
     case TARGET_SYS_READC:
-        qemu_log_mask(LOG_UNIMP, "%s: SYS_READC not implemented", __func__);
-        return 0;
+        return qemu_semihosting_console_inc(env);
     case TARGET_SYS_ISTTY:
         GET_ARG(0);
 

vl.c

@@ -4238,6 +4238,9 @@ int main(int argc, char **argv, char **envp)
     qemu_opts_foreach(qemu_find_opts("mon"),
                       mon_init_func, NULL, &error_fatal);
 
+    /* connect semihosting console input if requested */
+    qemu_semihosting_console_init();
+
     if (foreach_device_config(DEV_SERIAL, serial_parse) < 0)
         exit(1);
     if (foreach_device_config(DEV_PARALLEL, parallel_parse) < 0)