arch/xtensa: Add XTENSA_CALL0_ABI kconfig

New ABI, much simpler, slightly bigger/slower code, but MUCH cleaner
syscall entry.

Signed-off-by: Andy Ross <andyross@google.com>

Author: andyross

arch/xtensa/Kconfig

@@ -14,6 +14,20 @@ config SIMULATOR_XTENSA
 	help
 	  Specify if the board configuration should be treated as a simulator.
 
+config XTENSA_CALL0_ABI
+	bool "Use non-windowed ABI"
+	help
+	  When selected, all code will be built to use the CALL0 ABI
+	  instead of register windows (though the context management
+	  code will still exploit the extra registers for faster
+	  interrupt/exception handling -- this does not build a Zephyr
+	  to run on hardware that truly lacks register windows).  This
+	  is required for userspace, as exception handling across a
+	  priviledge boundary must be able to spill the full context.
+	  For other systems, it improves context switch speed and
+	  stack usage at the expense of somewhat higher code size and
+	  execution time for function calls.
+
 config SYS_CLOCK_HW_CYCLES_PER_SEC
 	prompt "Hardware clock cycles per second, 2000000 for ISS"
 	default 2000000

cmake/compiler/gcc/target.cmake

@@ -78,6 +78,10 @@ elseif("${ARCH}" STREQUAL "mips")
   include(${CMAKE_CURRENT_LIST_DIR}/target_mips.cmake)
 endif()
 
+if(CONFIG_XTENSA_CALL0_ABI)
+  list(APPEND TOOLCHAIN_C_FLAGS -mabi=call0)
+endif()
+
 if(SYSROOT_DIR)
   # The toolchain has specified a sysroot dir, pass it to the compiler
   list(APPEND TOOLCHAIN_C_FLAGS

cmake/toolchain/xcc/common.cmake

@@ -22,4 +22,8 @@ set(NOSYSDEF_CFLAG "")
 
 list(APPEND TOOLCHAIN_C_FLAGS -fms-extensions)
 
+if(CONFIG_XTENSA_CALL0_ABI)
+  list(APPEND TOOLCHAIN_C_FLAGS -mabi=call0)
+endif()
+
 set(TOOLCHAIN_HAS_NEWLIB OFF CACHE BOOL "True if toolchain supports newlib")