[PATCH v2 3/5] semihosting: add qemu_semihosting_console_inc for SYS_READC

["Alex Bennée" <alex.bennee@linaro.org>]

From: Keith Packard <keithp@keithp.com>

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>

---
v8
  - allow linux-user to block using unbuffered IO
  - spelling and wording fixes
v7
  - reword commit title
  - remove mutexs, halt CPU until data available
  - document cpu_loop_exit behavior in function API

squash! semihosting: add qemu_semihosting_console_inc for SYS_READC
---
 include/hw/semihosting/console.h  | 16 +++++++
 include/hw/semihosting/semihost.h |  4 ++
 hw/semihosting/console.c          | 79 +++++++++++++++++++++++++++++++
 linux-user/arm/semihost.c         | 27 +++++++++++
 stubs/semihost.c                  |  4 ++
 target/arm/arm-semi.c             |  3 +-
 vl.c                              |  3 ++
 7 files changed, 134 insertions(+), 2 deletions(-)

diff --git a/include/hw/semihosting/console.h b/include/hw/semihosting/console.h
index 9be9754bcdf..0238f540f4b 100644
--- a/include/hw/semihosting/console.h
+++ b/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
diff --git a/include/hw/semihosting/semihost.h b/include/hw/semihosting/semihost.h
index 60fc42d851e..b8ce5117ae0 100644
--- a/include/hw/semihosting/semihost.h
+++ b/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 */
diff --git a/hw/semihosting/console.c b/hw/semihosting/console.c
index b4b17c8afbc..6346bd7f506 100644
--- a/hw/semihosting/console.c
+++ b/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);
+    }
+}
diff --git a/linux-user/arm/semihost.c b/linux-user/arm/semihost.c
index a16b525eec0..a1f0f6050ea 100644
--- a/linux-user/arm/semihost.c
+++ b/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;
+}
diff --git a/stubs/semihost.c b/stubs/semihost.c
index f90589259c0..1d8b37f7b2f 100644
--- a/stubs/semihost.c
+++ b/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)
+{
+}
diff --git a/target/arm/arm-semi.c b/target/arm/arm-semi.c
index 6f7b6d801bf..47d61f6fe1f 100644
--- a/target/arm/arm-semi.c
+++ b/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);
 
diff --git a/vl.c b/vl.c
index 94508300c3c..1912f87822b 100644
--- a/vl.c
+++ b/vl.c
@@ -4142,6 +4142,9 @@ 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)
-- 
2.20.1