arm64: tlb: Rewrite stale comment in asm/tlbflush.h

Peter Z asked me to justify the barrier usage in asm/tlbflush.h, but
actually that whole block comment needs to be rewritten.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/tlbflush.h

@@ -70,43 +70,73 @@
 	})
 
 /*
- *	TLB Management
- *	==============
+ *	TLB Invalidation
+ *	================
  *
- *	The TLB specific code is expected to perform whatever tests it needs
- *	to determine if it should invalidate the TLB for each call.  Start
- *	addresses are inclusive and end addresses are exclusive; it is safe to
- *	round these addresses down.
+ * 	This header file implements the low-level TLB invalidation routines
+ *	(sometimes referred to as "flushing" in the kernel) for arm64.
  *
- *	flush_tlb_all()
+ *	Every invalidation operation uses the following template:
+ *
+ *	DSB ISHST	// Ensure prior page-table updates have completed
+ *	TLBI ...	// Invalidate the TLB
+ *	DSB ISH		// Ensure the TLB invalidation has completed
+ *      if (invalidated kernel mappings)
+ *		ISB	// Discard any instructions fetched from the old mapping
+ *
+ *
+ *	The following functions form part of the "core" TLB invalidation API,
+ *	as documented in Documentation/core-api/cachetlb.rst:
  *
- *		Invalidate the entire TLB.
+ *	flush_tlb_all()
+ *		Invalidate the entire TLB (kernel + user) on all CPUs
  *
  *	flush_tlb_mm(mm)
+ *		Invalidate an entire user address space on all CPUs.
+ *		The 'mm' argument identifies the ASID to invalidate.
+ *
+ *	flush_tlb_range(vma, start, end)
+ *		Invalidate the virtual-address range '[start, end)' on all
+ *		CPUs for the user address space corresponding to 'vma->mm'.
+ *		Note that this operation also invalidates any walk-cache
+ *		entries associated with translations for the specified address
+ *		range.
+ *
+ *	flush_tlb_kernel_range(start, end)
+ *		Same as flush_tlb_range(..., start, end), but applies to
+ * 		kernel mappings rather than a particular user address space.
+ *		Whilst not explicitly documented, this function is used when
+ *		unmapping pages from vmalloc/io space.
+ *
+ *	flush_tlb_page(vma, addr)
+ *		Invalidate a single user mapping for address 'addr' in the
+ *		address space corresponding to 'vma->mm'.  Note that this
+ *		operation only invalidates a single, last-level page-table
+ *		entry and therefore does not affect any walk-caches.
  *
- *		Invalidate all TLB entries in a particular address space.
- *		- mm	- mm_struct describing address space
  *
- *	flush_tlb_range(mm,start,end)
+ *	Next, we have some undocumented invalidation routines that you probably
+ *	don't want to call unless you know what you're doing:
  *
- *		Invalidate a range of TLB entries in the specified address
- *		space.
- *		- mm	- mm_struct describing address space
- *		- start - start address (may not be aligned)
- *		- end	- end address (exclusive, may not be aligned)
+ *	local_flush_tlb_all()
+ *		Same as flush_tlb_all(), but only applies to the calling CPU.
  *
- *	flush_tlb_page(vaddr,vma)
+ *	__flush_tlb_kernel_pgtable(addr)
+ *		Invalidate a single kernel mapping for address 'addr' on all
+ *		CPUs, ensuring that any walk-cache entries associated with the
+ *		translation are also invalidated.
  *
- *		Invalidate the specified page in the specified address range.
- *		- vaddr - virtual address (may not be aligned)
- *		- vma	- vma_struct describing address range
+ *	__flush_tlb_range(vma, start, end, stride, last_level)
+ *		Invalidate the virtual-address range '[start, end)' on all
+ *		CPUs for the user address space corresponding to 'vma->mm'.
+ *		The invalidation operations are issued at a granularity
+ *		determined by 'stride' and only affect any walk-cache entries
+ *		if 'last_level' is equal to false.
  *
- *	flush_kern_tlb_page(kaddr)
  *
- *		Invalidate the TLB entry for the specified page.  The address
- *		will be in the kernels virtual memory space.  Current uses
- *		only require the D-TLB to be invalidated.
- *		- kaddr - Kernel virtual memory address
+ *	Finally, take a look at asm/tlb.h to see how tlb_flush() is implemented
+ *	on top of these routines, since that is our interface to the mmu_gather
+ *	API as used by munmap() and friends.
  */
 static inline void local_flush_tlb_all(void)
 {