RISC-V: Add CONFIG_{NON,}PORTABLE

The RISC-V port has collected a handful of options that are
fundamentally non-portable.  To prevent users from shooting themselves
in the foot, hide them all behind a config entry that explicitly calls
out that non-portable binaries may be produced.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20220521193356.26562-1-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -223,6 +223,21 @@ source "arch/riscv/Kconfig.erratas"
 
 menu "Platform type"
 
+config NONPORTABLE
+	bool "Allow configurations that result in non-portable kernels"
+	help
+	  RISC-V kernel binaries are compatible between all known systems
+	  whenever possible, but there are some use cases that can only be
+	  satisfied by configurations that result in kernel binaries that are
+	  not portable between systems.
+
+	  Selecting N does not guarantee kernels will be portable to all known
+	  systems.  Selecting any of the options guarded by NONPORTABLE will
+	  result in kernel binaries that are unlikely to be portable between
+	  systems.
+
+	  If unsure, say N.
+
 choice
 	prompt "Base ISA"
 	default ARCH_RV64I
@@ -232,6 +247,7 @@ choice
 
 config ARCH_RV32I
 	bool "RV32I"
+	depends on NONPORTABLE
 	select 32BIT
 	select GENERIC_LIB_ASHLDI3
 	select GENERIC_LIB_ASHRDI3
@@ -552,6 +568,7 @@ config STACKPROTECTOR_PER_TASK
 
 config PHYS_RAM_BASE_FIXED
 	bool "Explicitly specified physical RAM address"
+	depends on NONPORTABLE
 	default n
 
 config PHYS_RAM_BASE
@@ -565,7 +582,7 @@ config PHYS_RAM_BASE
 
 config XIP_KERNEL
 	bool "Kernel Execute-In-Place from ROM"
-	depends on MMU && SPARSEMEM
+	depends on MMU && SPARSEMEM && NONPORTABLE
 	# This prevents XIP from being enabled by all{yes,mod}config, which
 	# fail to build since XIP doesn't support large kernels.
 	depends on !COMPILE_TEST
@@ -605,9 +622,16 @@ endmenu # "Boot options"
 
 config BUILTIN_DTB
 	bool
-	depends on OF
+	depends on OF && NONPORTABLE
 	default y if XIP_KERNEL
 
+config PORTABLE
+	bool
+	default !NONPORTABLE
+	select EFI
+	select OF
+	select MMU
+
 menu "Power management options"
 
 source "kernel/power/Kconfig"

arch/riscv/configs/32-bit.config

@@ -1,2 +1,4 @@
 CONFIG_ARCH_RV32I=y
 CONFIG_32BIT=y
+# CONFIG_PORTABLE is not set
+CONFIG_NONPORTABLE=y

arch/riscv/configs/nommu_k210_defconfig

@@ -28,6 +28,7 @@ CONFIG_EMBEDDED=y
 CONFIG_SLOB=y
 # CONFIG_MMU is not set
 CONFIG_SOC_CANAAN=y
+CONFIG_NONPORTABLE=y
 CONFIG_SMP=y
 CONFIG_NR_CPUS=2
 CONFIG_CMDLINE="earlycon console=ttySIF0"

arch/riscv/configs/nommu_k210_sdcard_defconfig

@@ -20,6 +20,7 @@ CONFIG_EMBEDDED=y
 CONFIG_SLOB=y
 # CONFIG_MMU is not set
 CONFIG_SOC_CANAAN=y
+CONFIG_NONPORTABLE=y
 CONFIG_SMP=y
 CONFIG_NR_CPUS=2
 CONFIG_CMDLINE="earlycon console=ttySIF0 root=/dev/mmcblk0p1 rootwait ro"

arch/riscv/configs/nommu_virt_defconfig

@@ -25,6 +25,7 @@ CONFIG_EXPERT=y
 CONFIG_SLOB=y
 # CONFIG_MMU is not set
 CONFIG_SOC_VIRT=y
+CONFIG_NONPORTABLE=y
 CONFIG_SMP=y
 CONFIG_CMDLINE="root=/dev/vda rw earlycon=uart8250,mmio,0x10000000,115200n8 console=ttyS0"
 CONFIG_CMDLINE_FORCE=y

arch/riscv/configs/rv32_defconfig

@@ -18,6 +18,7 @@ CONFIG_EXPERT=y
 CONFIG_PROFILING=y
 CONFIG_SOC_SIFIVE=y
 CONFIG_SOC_VIRT=y
+CONFIG_NONPORTABLE=y
 CONFIG_ARCH_RV32I=y
 CONFIG_SMP=y
 CONFIG_HOTPLUG_CPU=y