arm64: tlbflush: avoid flushing when fullmm == 1

The TLB gather code sets fullmm=1 when tearing down the entire address
space for an mm_struct on exit or execve. Given that the ASID allocator
will never re-allocate a dirty ASID, this flushing is not needed and can
simply be avoided in the flushing code.
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/tlb.h

@@ -37,17 +37,21 @@ static inline void __tlb_remove_table(void *_table)
 
 static inline void tlb_flush(struct mmu_gather *tlb)
 {
-	if (tlb->fullmm) {
-		flush_tlb_mm(tlb->mm);
-	} else {
 	struct vm_area_struct vma = { .vm_mm = tlb->mm, };
+
+	/*
+	 * The ASID allocator will either invalidate the ASID or mark
+	 * it as used.
+	 */
+	if (tlb->fullmm)
+		return;
+
 	/*
 	 * The intermediate page table levels are already handled by
 	 * the __(pte|pmd|pud)_free_tlb() functions, so last level
 	 * TLBI is sufficient here.
 	 */
 	__flush_tlb_range(&vma, tlb->start, tlb->end, true);
-	}
 }
 
 static inline void __pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte,