sfence

sfence is a CPU instruction that acts as a memory fence or barrier. Its primary purpose is to control the order in which memory operations are observed by different processing units or by peripherals. Specifically, an sfence instruction ensures that all memory writes that occurred before the sfence are completed and visible to other observers before any memory reads that occur after the sfence. This prevents certain reordering of memory operations by the processor that could lead to incorrect program behavior, particularly in multi-processor systems or when interacting with I/O devices.

In essence, sfence provides a synchronization point for memory operations. Without sfence, a processor might optimize performance by executing memory writes and reads in an order that is not strictly sequential from the perspective of external entities. This can be problematic when one processor needs to signal another processor about a change in memory or when a device driver needs to ensure that data written by the CPU is actually present in memory before the device attempts to access it. The sfence instruction guarantees that by the time the sfence instruction completes, all preceding writes are visible. It does not affect the ordering of reads with respect to writes or reads with respect to reads.