[Bug 104100] Nearly 10% performance drop in SynMark CSCloth from "i965: Emit CS stall before MEDIA_VFE_STATE"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=104100

            Bug ID: 104100
           Summary: Nearly 10% performance drop in SynMark CSCloth from
                    "i965: Emit CS stall before MEDIA_VFE_STATE"
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: intel-3d-bugs at lists.freedesktop.org
          Reporter: eero.t.tamminen at intel.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

Following commit drops performance in SynMark CSCloth test, as expected:
-----------------------------------------------------
commit 55a97db52347f62111a24715078c6035380d3e19
Author:     Kenneth Graunke <kenneth at whitecape.org>
AuthorDate: Tue Oct 31 09:57:54 2017 -0700
Commit:     Matt Turner <mattst88 at gmail.com>
CommitDate: Mon Dec 4 10:02:46 2017 -0800

    i965: Emit CS stall before MEDIA_VFE_STATE.

    This fixes hangs on GFXBench 5's Aztec Ruins benchmark.

    Unfortunately, it regresses OglCSCloth performance by about 10%. There
    are some ideas for fixing that.

    The Vulkan driver already emits this stall.

    Reviewed-by: Matt Turner <mattst88 at gmail.com>
-----------------------------------------------------

The drop is:
* 8-9% on SKL GT4e
* 5-6% on KBL GT3e
* 3-4% on SKL & KBL GT2
* within (largish) variance on BDW GT2 & GT3

On HSW GT it would seem to actually improve perf marginally.

Latter is interesting because Kenneth's trial fix for the performance
regression (verified to fix it):
https://cgit.freedesktop.org/~kwg/mesa/commit/?h=vfe-stall-3&id=ab9bff0ac43f0d878106ba124e84edca9b359c8a

Is according to Kenneth causing problems on HSW.  Maybe it could be applied
only on newer GENs?

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