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[Leann Ogasawara <leann.ogasawara@xxxxxxxxxxxxx>]

** Changed in: intel
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1084377

Title:
  [Feature] non-lazy extended state restore during process context
  switch

Status in intel:
  Fix Released
Status in “linux” package in Ubuntu:
  Fix Released
Status in “linux” source package in Raring:
  Fix Released

Bug description:
  Fundamental model of the current Linux kernel is to lazily restore extended-state (xstate like FP/SSE/AVX) during context switch. On the first use of the xstate after context-switch, application takes a #DNA exception and the kernel will restore it. After 5 such consecutive xstate usages post task-switch, kernel adopts a heuristic to pre-load the xstate during context-switch for the next 251 task-switches.  
  This feature changes that fundamental lazy model to the non-lazy model for the processors supporting any new xstate feature beyond AVX (In essence making the pre-load heuristic as the fundamental model). 

  While this is a must for enabling some of the new features in the next
  generation processor after Broadwell, as we saw some performance
  improvements (like avoiding unnecessary DNA exceptions, improving the
  kernel code that uses fpu inside the kernel etc), we enabled this
  behavior for all cpu's supporting xsaveopt. So it will be from SNB.

  Upstream status:
  Code in 3.7-rc1:
  050902c011712ad4703038fa4489ec4edd87d396
  0ca5bd0d886578ad0afeceaa83458c0f35cb3c6b
  72a671ced66db6d1c2bfff1c930a101ac8d08204
  e962591749dfd4df9fea2c530ed7a3cfed50e5aa
  377ffbcc536a5a6666dc077395163ab149c02610
  9c1c3fac53378c9782c18f80107965578d7b7167
  841e3604d35aa70d399146abdc526d8c89a2c2f5
  9c6ff8bbb69a4e7b47ac40bfa44509296e89c5c0
  304bceda6a18ae0b0240b8aac9a6bdf8ce2d2469
  5d2bd7009f306c82afddd1ca4d9763ad8473c216
  212b02125f3725127148475059a70031bd031bdb
  e00229819f306b1f86134095347e9187dc346bd1
  a8615af4bc3621cb01096541dafa6f68352ec2d9

  
  Some data on a two socket SNB system:
   * Saved 20K DNA exceptions during boot on a two socket SNB system.
   * Saved 50K DNA exceptions during kernel-compilation workload.
   * Improved throughput of the AVX based checksumming function inside the
     kernel by ~15% as xsave/xrstor is faster than the serializing clts/stts
     pair.
  DNA stands for (Device Not Available).

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