irqchip/riscv: Ensure ordering of memory writes and IPI writes

irqchip/riscv: Ensure ordering of memory writes and IPI writes

RISC-V distinguishes between memory accesses and device I/O and uses FENCE
instruction to order them as viewed by other RISC-V harts and external
devices or coprocessors. The FENCE instruction can order any combination of
device input(I), device output(O), memory reads(R) and memory
writes(W). For example, 'fence w, o' is used to ensure all memory writes
from instructions preceding the FENCE instruction appear earlier in the
global memory order than device output writes from instructions after the
FENCE instruction.

RISC-V issues IPIs by writing to the IMSIC/ACLINT MMIO registers, which is
regarded as device output operation. However, the existing implementation
of the IMSIC/ACLINT drivers issue the IPI via writel_relaxed(), which does
not guarantee the order of device output operation and preceding memory
writes. As a consequence the hart receiving the IPI might not observe the
IPI related data.

Fix this by replacing writel_relaxed() with writel() when issuing IPIs,
which uses 'fence w, o' to ensure all previous writes made by the current
hart are visible to other harts before they receive the IPI.

Signed-off-by: Xu Lu <luxu.kernel@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250127093846.98625-1-luxu.kernel@bytedance.com