The previous patches added arch support matrices for more than 40 generic kernel features
that need per architecture support.

The structure of the feature descriptions is the following:

Each feature has its own directory under Documentation/features/subsystem_name/feature_name/,
and the arch-support.txt file shows its current arch porting status.

For example, lockdep support is shown the following way:

    triton:~/tip> cat Documentation/features/locking/lockdep/arch-support.txt
    #
    # Feature name:          lockdep
    #         Kconfig:       LOCKDEP_SUPPORT
    #         description:   arch supports the runtime locking correctness debug facility
    #
        -----------------------
        |         arch |status|
        -----------------------
        |       alpha: | TODO |
        |         arc: |  ok  |
        |         arm: |  ok  |
        |       arm64: |  ok  |
        |       avr32: |  ok  |
        |    blackfin: |  ok  |
        |         c6x: | TODO |
        |        cris: | TODO |
        |         frv: | TODO |
        |       h8300: | TODO |
        |     hexagon: |  ok  |
        |        ia64: | TODO |
        |        m32r: | TODO |
        |        m68k: | TODO |
        |       metag: |  ok  |
        |  microblaze: |  ok  |
        |        mips: |  ok  |
        |     mn10300: | TODO |
        |       nios2: | TODO |
        |    openrisc: | TODO |
        |      parisc: | TODO |
        |     powerpc: |  ok  |
        |        s390: |  ok  |
        |       score: |  ok  |
        |          sh: |  ok  |
        |       sparc: |  ok  |
        |        tile: |  ok  |
        |          um: |  ok  |
        |   unicore32: |  ok  |
        |         x86: |  ok  |
        |      xtensa: |  ok  |
        -----------------------

For generic kernel features that need architecture support, the
arch-support.txt file in each feature directory shows the arch
support matrix, for all upstream Linux architectures.

The meaning of entries in the tables is:

    | ok |  # feature supported by the architecture
    |TODO|  # feature not yet supported by the architecture
    | .. |  # feature cannot be supported by the hardware

This directory structure can be used in the future to add other
files - such as porting guides, testing description, etc.

The Documentation/features/ hierarchy may also include generic
kernel features that works on every architecture, in that case
the arch-support.txt file will list every architecture as
supported.

To list an architecture's unsupported features, just do something
like:

  triton:~/tip>  git grep -lE 'x86.*TODO' Documentation/features/*/*/arch-support.txt
  Documentation/features/lib/strncasecmp/arch-support.txt
  Documentation/features/time/arch-tick-broadcast/arch-support.txt

which will print the list of not yet supported features.

The Documentation/features/list-arch.sh script will print the current
support matrix of one architecture:

  triton:~/tip> Documentation/features/list-arch.sh
  #
  # Kernel feature support matrix of the 'x86' architecture:
  #
      core/ BPF-JIT              :  ok  |                        HAVE_BPF_JIT #  arch supports BPF JIT optimizations
      core/ generic-idle-thread  :  ok  |             GENERIC_SMP_IDLE_THREAD #  arch makes use of the generic SMP idle thread facility
      core/ jump-labels          :  ok  |                HAVE_ARCH_JUMP_LABEL #  arch supports live patched, high efficiency branches
      core/ tracehook            :  ok  |                 HAVE_ARCH_TRACEHOOK #  arch supports tracehook (ptrace) register handling APIs
     debug/ gcov-profile-all     :  ok  |           ARCH_HAS_GCOV_PROFILE_ALL #  arch supports whole-kernel GCOV code coverage profiling
     debug/ KASAN                :  ok  |                     HAVE_ARCH_KASAN #  arch supports the KASAN runtime memory checker
     debug/ kgdb                 :  ok  |                      HAVE_ARCH_KGDB #  arch supports the kGDB kernel debugger
     debug/ kprobes              :  ok  |                        HAVE_KPROBES #  arch supports live patched kernel probe
     debug/ kprobes-on-ftrace    :  ok  |              HAVE_KPROBES_ON_FTRACE #  arch supports combined kprobes and ftrace live patching
     debug/ kretprobes           :  ok  |                     HAVE_KRETPROBES #  arch supports kernel function-return probes
     debug/ optprobes            :  ok  |                      HAVE_OPTPROBES #  arch supports live patched optprobes
     debug/ stackprotector       :  ok  |              HAVE_CC_STACKPROTECTOR #  arch supports compiler driven stack overflow protection
     debug/ uprobes              :  ok  |               ARCH_SUPPORTS_UPROBES #  arch supports live patched user probes
     debug/ user-ret-profiler    :  ok  |           HAVE_USER_RETURN_NOTIFIER #  arch supports user-space return from system call profiler
        io/ dma-api-debug        :  ok  |                  HAVE_DMA_API_DEBUG #  arch supports DMA debug facilities
        io/ dma-contiguous       :  ok  |                 HAVE_DMA_CONTIGUOUS #  arch supports the DMA CMA (continuous memory allocator)
        io/ dma_map_attrs        :  ok  |                      HAVE_DMA_ATTRS #  arch provides dma_*map*_attrs() APIs
        io/ sg-chain             :  ok  |                   ARCH_HAS_SG_CHAIN #  arch supports chained scatter-gather lists
       lib/ strncasecmp          : TODO |             __HAVE_ARCH_STRNCASECMP #  arch provides an optimized strncasecmp() function
   locking/ cmpxchg-local        :  ok  |                  HAVE_CMPXCHG_LOCAL #  arch supports the this_cpu_cmpxchg() API
   locking/ lockdep              :  ok  |                     LOCKDEP_SUPPORT #  arch supports the runtime locking correctness debug facility
   locking/ queued-rwlocks       :  ok  |             ARCH_USE_QUEUED_RWLOCKS #  arch supports queued rwlocks
   locking/ queued-spinlocks     :  ok  |           ARCH_USE_QUEUED_SPINLOCKS #  arch supports queued spinlocks
   locking/ rwsem-optimized      :  ok  |               Optimized asm/rwsem.h #  arch provides optimized rwsem APIs
      perf/ kprobes-event        :  ok  |      HAVE_REGS_AND_STACK_ACCESS_API #  arch supports kprobes with perf events
      perf/ perf-regs            :  ok  |                      HAVE_PERF_REGS #  arch supports perf events register access
      perf/ perf-stackdump       :  ok  |           HAVE_PERF_USER_STACK_DUMP #  arch supports perf events stack dumps
     sched/ numa-balancing       :  ok  |        ARCH_SUPPORTS_NUMA_BALANCING #  arch supports NUMA balancing
   seccomp/ seccomp-filter       :  ok  |            HAVE_ARCH_SECCOMP_FILTER #  arch supports seccomp filters
      time/ arch-tick-broadcast  : TODO |             ARCH_HAS_TICK_BROADCAST #  arch provides tick_broadcast()
      time/ clockevents          :  ok  |                 GENERIC_CLOCKEVENTS #  arch support generic clock events
      time/ context-tracking     :  ok  |               HAVE_CONTEXT_TRACKING #  arch supports context tracking for NO_HZ_FULL
      time/ irq-time-acct        :  ok  |            HAVE_IRQ_TIME_ACCOUNTING #  arch supports precise IRQ time accounting
      time/ modern-timekeeping   :  ok  |            !ARCH_USES_GETTIMEOFFSET #  arch does not use arch_gettimeoffset() anymore
      time/ virt-cpuacct         :  ok  |            HAVE_VIRT_CPU_ACCOUNTING #  arch supports precise virtual CPU time accounting
        vm/ ELF-ASLR             :  ok  |              ARCH_HAS_ELF_RANDOMIZE #  arch randomizes the stack, heap and binary images of ELF binaries
        vm/ huge-vmap            :  ok  |                 HAVE_ARCH_HUGE_VMAP #  arch supports the ioremap_pud_enabled() and ioremap_pmd_enabled() VM APIs
        vm/ ioremap_prot         :  ok  |                   HAVE_IOREMAP_PROT #  arch has ioremap_prot()
        vm/ numa-memblock        :  ok  |              HAVE_MEMBLOCK_NODE_MAP #  arch supports NUMA aware memblocks
        vm/ PG_uncached          :  ok  |               ARCH_USES_PG_UNCACHED #  arch supports the PG_uncached page flag
        vm/ pmdp_splitting_flush :  ok  |    __HAVE_ARCH_PMDP_SPLITTING_FLUSH #  arch supports the pmdp_splitting_flush() VM API
        vm/ pte_special          :  ok  |             __HAVE_ARCH_PTE_SPECIAL #  arch supports the pte_special()/pte_mkspecial() VM APIs
        vm/ THP                  :  ok  |      HAVE_ARCH_TRANSPARENT_HUGEPAGE #  arch supports transparent hugepages

Cc: <linux-api@vger.kernel.org>
Cc: <linux-arch@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>