[PATCH] riscv: Add WFI to secondary hart spinwait loop

[Adriano Vero]

The .Lwait_for_cpu_up loop in the RISCV_BOOT_SPINWAIT path busy-polls
__cpu_spinwait_stack_pointer and __cpu_spinwait_task_pointer, burning
power on all non-boot harts while they wait for the primary hart to
complete early boot setup.

Add a WFI instruction before each polling iteration to allow the
hardware to enter a low-power state while waiting. Per the RISC-V
privileged specification, WFI wakes on any pending interrupt even
with global interrupts disabled (SIE=0), and implementations are
permitted to treat it as a NOP, so this is safe in all contexts.

The same pattern is already used in .Lsecondary_park in the same
file.

Signed-off-by: Adriano Vero <litaliano00.contact@xxxxxxxxx>
---
 arch/riscv/kernel/head.S | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 9c99c5ad6..ca208da7c 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -385,7 +385,14 @@ SYM_CODE_START(_start_kernel)
 	 * get far enough along the boot process that it should continue.
 	 */
 .Lwait_for_cpu_up:
-	/* FIXME: We should WFI to save some energy here. */
+	/*
+	 * Wait for the boot hart to populate the stack and task pointers.
+	 * Use WFI to avoid burning power in a busy-wait loop. Per the
+	 * RISC-V privileged spec, WFI wakes on a pending interrupt even
+	 * with global interrupts disabled (e.g. SIE=0), and implementations
+	 * are permitted to treat it as a NOP, so this is always safe.
+	 */
+	wfi
 	REG_L sp, (a1)
 	REG_L tp, (a2)
 	beqz sp, .Lwait_for_cpu_up
-- 
2.53.0