TLBflush

TLBflush is the operation of invalidating entries in a CPU's Translation Lookaside Buffer (TLB) to ensure that memory references use current address translations. The TLB caches recent virtual-to-physical mappings produced by the page table, so when page tables are modified, unmapped memory is made inaccessible, or permissions change, stale entries must be discarded to preserve correctness and memory protection.

Flushing can be global, removing all TLB entries, or selective, invalidating a single page, a range of addresses, or an address space. In single-core environments, software or hardware can perform cache invalidations efficiently. In multi-core or multi-processor systems, a TLB shootdown is typically used: a coordination mechanism (often via inter-processor interrupts) ensures that all CPUs invalidate the relevant TLB entries for a given process or context.

Architecture-specific behavior varies. On x86, the CR3 register controls the active page table, and reloading CR3 or issuing per-page invalidation (such as invlpg for a single page) triggers a TLB flush. Some processors support global invalidation methods or ASID-based tagging to reduce unnecessary flushes. ARM architectures provide TLBI instructions to invalidate TLBs by ASID or domain. Other architectures, like MIPS and PowerPC, have analogous mechanisms for issuing TLB invalidations.

Impact and practice: TLB flushes can incur performance penalties due to increased TLB misses after invalidation. Operating systems strive to minimize flush frequency, batching updates, and using per-context or per-address-space invalidations where possible. Virtualization adds complexity, as nested translations may require coordinated invalidations across layers.

In summary, TLBflush is a critical mechanism for maintaining correct and secure memory translations following page table changes, with implementations and performance characteristics that vary by architecture.