semihosting: introduce CONFIG_SEMIHOSTING

There isn't much point building semihosting for platforms that don't
support it. Introduce a new symbol and enable it only for the softmmu
targets that need it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

default-configs/arm-softmmu.mak

@@ -39,3 +39,4 @@ CONFIG_MICROBIT=y
 CONFIG_FSL_IMX25=y
 CONFIG_FSL_IMX7=y
 CONFIG_FSL_IMX6UL=y
+CONFIG_SEMIHOSTING=y

default-configs/lm32-softmmu.mak

@@ -4,6 +4,8 @@
 #
 #CONFIG_MILKYMIST_TMU2=n        # disabling it actually causes compile-time failures
 
+CONFIG_SEMIHOSTING=y
+
 # Boards:
 #
 CONFIG_LM32=y

default-configs/m68k-softmmu.mak

@@ -1,5 +1,7 @@
 # Default configuration for m68k-softmmu
 
+CONFIG_SEMIHOSTING=y
+
 # Boards:
 #
 CONFIG_AN5206=y

default-configs/mips-softmmu-common.mak

@@ -35,6 +35,7 @@ CONFIG_MIPS_CPS=y
 CONFIG_MIPS_ITU=y
 CONFIG_R4K=y
 CONFIG_MALTA=y
+CONFIG_SEMIHOSTING=y
 CONFIG_PCNET_PCI=y
 CONFIG_MIPSSIM=y
 CONFIG_ACPI_SMBUS=y

default-configs/nios2-softmmu.mak

@@ -1,5 +1,7 @@
 # Default configuration for nios2-softmmu
 
+CONFIG_SEMIHOSTING=y
+
 # Boards:
 #
 CONFIG_NIOS2_10M50=y

default-configs/xtensa-softmmu.mak

@@ -1,5 +1,7 @@
 # Default configuration for Xtensa
 
+CONFIG_SEMIHOSTING=y
+
 # Boards:
 #
 CONFIG_XTENSA_SIM=y

hw/Kconfig

@@ -29,6 +29,7 @@ source pci/Kconfig
 source rdma/Kconfig
 source scsi/Kconfig
 source sd/Kconfig
+source semihosting/Kconfig
 source smbios/Kconfig
 source ssi/Kconfig
 source timer/Kconfig

hw/semihosting/Kconfig

@@ -0,0 +1,3 @@
+
+config SEMIHOSTING
+       bool

hw/semihosting/Makefile.objs

@@ -1 +1 @@
-common-obj-$(CONFIG_SOFTMMU) += config.o
+obj-$(CONFIG_SEMIHOSTING) += config.o

stubs/Makefile.objs

@@ -40,3 +40,4 @@ stub-obj-y += pci-host-piix.o
 stub-obj-y += ram-block.o
 stub-obj-y += ramfb.o
 stub-obj-y += fw_cfg.o
+stub-obj-$(CONFIG_SOFTMMU) += semihost.o

stubs/semihost.c

@@ -0,0 +1,66 @@
+/*
+ * Semihosting Stubs for SoftMMU
+ *
+ * Copyright (c) 2019 Linaro Ltd
+ *
+ * Stubs for SoftMMU targets that don't actually do semihosting.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/option.h"
+#include "qemu/error-report.h"
+#include "hw/semihosting/semihost.h"
+
+/* Empty config */
+QemuOptsList qemu_semihosting_config_opts = {
+    .name = "",
+    .head = QTAILQ_HEAD_INITIALIZER(qemu_semihosting_config_opts.head),
+    .desc = {
+        { /* end of list */ }
+    },
+};
+
+/* Queries to config status default to off */
+bool semihosting_enabled(void)
+{
+    return false;
+}
+
+SemihostingTarget semihosting_get_target(void)
+{
+    return SEMIHOSTING_TARGET_AUTO;
+}
+
+/*
+ * All the rest are empty subs. We could g_assert_not_reached() but
+ * that adds extra weight to the final binary. Waste not want not.
+ */
+void qemu_semihosting_enable(void)
+{
+}
+
+int qemu_semihosting_config_options(const char *optarg)
+{
+    return 1;
+}
+
+const char *semihosting_get_arg(int i)
+{
+    return NULL;
+}
+
+int semihosting_get_argc(void)
+{
+    return 0;
+}
+
+const char *semihosting_get_cmdline(void)
+{
+    return NULL;
+}
+
+void semihosting_arg_fallback(const char *file, const char *cmd)
+{
+}