configure: allow the selection of alternate config in the build

While the default config works well enough it does end up enabling a
lot of stuff. For more minimal builds we can select a different list
of devices and let Kconfig work out what we want. For example:

  ../../configure --without-default-features \
    --target-list=arm-softmmu,aarch64-softmmu \
    --with-devices-aarch64=minimal

will override the aarch64-softmmu default set of devices with a more
minimal set of devices that just enables the virt and sbsa-ref models.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210707131744.26027-6-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configs/devices/aarch64-softmmu/minimal.mak

@@ -0,0 +1,9 @@
+#
+# A minimal version of the config that only supports only a few
+# virtual machines. This avoids bringing in any of numerous legacy
+# features from the 32bit platform (although virt still supports 32bit
+# itself)
+#
+
+CONFIG_ARM_VIRT=y
+CONFIG_SBSA_REF=y

configure

@@ -915,6 +915,18 @@ for opt do
   ;;
   --without-default-devices) default_devices="false"
   ;;
+  --with-devices-*[!a-zA-Z0-9_-]*=*) error_exit "Passed bad --with-devices-FOO option"
+  ;;
+  --with-devices-*) device_arch=${opt#--with-devices-};
+                    device_arch=${device_arch%%=*}
+                    cf=$source_path/configs/devices/$device_arch-softmmu/$optarg.mak
+                    if test -f "$cf"; then
+                        device_archs="$device_archs $device_arch"
+                        eval "devices_${device_arch}=\$optarg"
+                    else
+                        error_exit "File $cf does not exist"
+                    fi
+  ;;
   --without-default-features) # processed above
   ;;
   --enable-gprof) gprof="yes"
@@ -1764,7 +1776,8 @@ Advanced options (experts only):
   --without-default-features default all --enable-* options to "disabled"
   --without-default-devices  do not include any device that is not needed to
                            start the emulator (only use if you are including
-                           desired devices in default-configs/devices/)
+                           desired devices in configs/devices/)
+  --with-devices-ARCH=NAME override default configs/devices
   --enable-debug           enable common debug build options
   --enable-sanitizers      enable default sanitizers
   --enable-tsan            enable thread sanitizer
@@ -5082,6 +5095,15 @@ if test "$skip_meson" = no; then
 
   echo "# Automatically generated by configure - do not modify" > $cross
   echo "[properties]" >> $cross
+
+  # unroll any custom device configs
+  if test -n "$device_archs"; then
+      for a in $device_archs; do
+          eval "c=\$devices_${a}"
+          echo "${a}-softmmu = '$c'" >> $cross
+      done
+  fi
+
   test -z "$cxx" && echo "link_language = 'c'" >> $cross
   echo "[built-in options]" >> $cross
   echo "c_args = [${CFLAGS:+$(meson_quote $CFLAGS)}]" >> $cross

meson.build

@@ -1595,9 +1595,10 @@ foreach target : target_dirs
                                                configuration: config_target_data)}
 
   if target.endswith('-softmmu')
+    config_input = meson.get_external_property(target, 'default')
     config_devices_mak = target + '-config-devices.mak'
     config_devices_mak = configure_file(
-      input: ['configs/devices' / target / 'default.mak', 'Kconfig'],
+      input: ['configs/devices' / target / config_input + '.mak', 'Kconfig'],
       output: config_devices_mak,
       depfile: config_devices_mak + '.d',
       capture: true,