src: Consolidate x86_64 support Kconfig

Introduce `USE_EXP_X86_64_SUPPORT` in `src/arch/x86/Kconfig` and guard
it with `HAVE_EXP_X86_64_SUPPORT`. Replace the per-CPU implementations
of the same functionality with the newly-added Kconfig options. Update
documentation and the config file for QEMU accordingly.

Change-Id: I550216fd2a8323342d6b605306b0b95ffd5dcd1c
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55760
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/mainboard/emulation/qemu-i440fx.md

@@ -15,7 +15,7 @@ processor supports x86_64 instructions (long mode).
 The qemu-i440fx mainboard has been ported to x86_64 and will serve as
 reference platform to enable additional platforms.
 
-To enable the support set the Kconfig option ``CONFIG_CPU_QEMU_X86_64=y``.
+To enable the support set the Kconfig option ``CONFIG_USE_EXP_X86_64_SUPPORT=y``.
 
 ## Installing qemu
 

Documentation/mainboard/emulation/qemu-q35.md

@@ -15,7 +15,7 @@ processor supports x86_64 instructions (long mode).
 The qemu-q35 mainboard has been ported to x86_64 and will serve as
 reference platform to enable additional platforms.
 
-To enable the support set the Kconfig option ``CONFIG_CPU_QEMU_X86_64=y``.
+To enable the support set the Kconfig option ``CONFIG_USE_EXP_X86_64_SUPPORT=y``.
 
 ## Installing qemu
 

configs/config.emulation_qemu_x86_i440fx_x86_64

@@ -1 +1 @@
-CONFIG_CPU_QEMU_X86_64=y
+CONFIG_USE_EXP_X86_64_SUPPORT=y

src/arch/x86/Kconfig

@@ -63,6 +63,24 @@ config ARCH_ALL_STAGES_X86
 	bool
 	default y
 
+config HAVE_EXP_X86_64_SUPPORT
+	bool
+	help
+	  Enable experimental support to build and run coreboot in 64-bit mode.
+	  When selecting this option for a new platform, it is highly advisable
+	  to provide a config file for Jenkins to build-test the 64-bit option.
+
+config USE_EXP_X86_64_SUPPORT
+	bool "[EXPERIMENTAL] Run coreboot in long (64-bit) mode"
+	depends on HAVE_EXP_X86_64_SUPPORT
+	select ARCH_ALL_STAGES_X86_64
+	help
+	  When set, most of coreboot runs in long (64-bit) mode instead of the
+	  usual protected flat (32-bit) mode. 64-bit CPUs and OSes can be used
+	  irrespective of whether coreboot runs in 32-bit or 64-bit mode. This
+	  is an experimental option: do not enable unless one wants to test it
+	  and has the means to recover a system when coreboot fails to boot.
+
 config ARCH_X86_64_PGTBL_LOC
 	hex "x86_64 page table location in CBFS"
 	depends on ARCH_BOOTBLOCK_X86_64

src/cpu/intel/model_206ax/Kconfig

@@ -3,16 +3,11 @@ config CPU_INTEL_MODEL_206AX
 
 if CPU_INTEL_MODEL_206AX
 
-config ARCH_EXP_X86_64
-	bool "Experimental 64bit support"
-	depends on USE_NATIVE_RAMINIT
-	default n
-
 config CPU_SPECIFIC_OPTIONS
 	def_bool y
-	select ARCH_ALL_STAGES_X86_64 if ARCH_EXP_X86_64
 	select ARCH_X86
 	select BOOT_DEVICE_SPI_FLASH_NO_EARLY_WRITES
+	select HAVE_EXP_X86_64_SUPPORT if USE_NATIVE_RAMINIT
 	select MMX
 	select SSE2
 	select UDELAY_TSC

src/cpu/qemu-x86/Kconfig

@@ -3,6 +3,7 @@
 config CPU_QEMU_X86
 	bool
 	select ARCH_X86
+	select HAVE_EXP_X86_64_SUPPORT
 	select UDELAY_TSC
 	select TSC_MONOTONIC_TIMER
 	select UNKNOWN_TSC_RATE
@@ -49,12 +50,4 @@ config MAX_CPUS
 	default 32 if SMM_TSEG
 	default 4
 
-config CPU_QEMU_X86_64
-	bool "Experimental 64bit support"
-	select ARCH_ALL_STAGES_X86_64
-
-config CPU_QEMU_X86_32
-	bool
-	default n if CPU_QEMU_X86_64
-	default y
 endif