RISC-V: Only default to spinwait on SBI-0.1 and M-mode

The spinwait boot method has been superseded by the SBI HSM extension
for some time now, but it still enabled by default.  This causes some
issues on large hart count systems, which will hang if a physical hart
exists that is larger than NR_CPUS.

Users on modern SBI implementation don't need spinwait, and while it's
probably possible to deal with some of the spinwait issues let's just
restrict the default to systems that are likely to actually use it.
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20220421170354.10555-1-palmer@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -396,7 +396,7 @@ config RISCV_SBI_V01
 config RISCV_BOOT_SPINWAIT
 	bool "Spinwait booting method"
 	depends on SMP
-	default y
+	default y if RISCV_SBI_V01 || RISCV_M_MODE
 	help
 	  This enables support for booting Linux via spinwait method. In the
 	  spinwait method, all cores randomly jump to Linux. One of the cores
@@ -407,6 +407,12 @@ config RISCV_BOOT_SPINWAIT
 	  rely on ordered booting via SBI HSM extension which gets chosen
 	  dynamically at runtime if the firmware supports it.
 
+	  Since spinwait is incompatible with sparse hart IDs, it requires
+	  NR_CPUS be large enough to contain the physical hart ID of the first
+	  hart to enter Linux.
+
+	  If unsure what to do here, say N.
+
 config KEXEC
 	bool "Kexec system call"
 	select KEXEC_CORE