• Imre Deak's avatar
    drm/i915/bxt: Fix inadvertent CPU snooping due to incorrect MOCS config · 6bee14ed
    Imre Deak authored
    Setting a write-back cache policy in the MOCS entry definition also
    implies snooping, which has a considerable overhead. This is
    unexpected for a few reasons:
    - From user-space's point of view since it didn't want a coherent
      surface (it didn't set the buffer as such via the set caching IOCTL).
    - There is a separate MOCS entry field for snooping (which we never
      set).
    - This MOCS table is about caching in (e)LLC and there is no (e)LLC on
      BXT. There is a separate table for L3 cache control.
    
    Considering the above the current behavior of snooping looks like an
    unintentional side-effect of the WB setting. Changing it to be LLC-UC
    gets rid of the snooping without any ill-effects. For a coherent
    surface the application would use a separate MOCS entry at index 1 and
    call the set caching IOCTL to setup the PTE entries for the
    corresponding buffer to be snooped. In the future we could also add a
    new MOCS entry for coherent surfaces.
    
    This resulted in 70% improvement in synthetic texturing benchmarks.
    
    Kudos to Valtteri Rantala, Eero Tamminen and Michael T Frederick and
    Ville who helped to narrow the source of problem to the kernel and to
    the snooping behaviour in particular.
    
    With a follow-up change to adjust the 3rd entry value
    igt/gem_mocs_settings is passing after this change.
    
    v2:
    - Rebase on v2 of patch 1/2.
    v3:
    - Set the entry as LLC uncached instead of PTE-passthrough. This way
      we also keep snooping disabled, but we also make the cacheability/
      coherency setting indepent of the PTE which is managed by the
      kernel. (Chris)
    
    CC: Rong R Yang <rong.r.yang@intel.com>
    CC: Yakui Zhao <yakui.zhao@intel.com>
    CC: Valtteri Rantala <valtteri.rantala@intel.com>
    CC: Eero Tamminen <eero.t.tamminen@intel.com>
    CC: Michael T Frederick <michael.t.frederick@intel.com>
    CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
    CC: Chris Wilson <chris@chris-wilson.co.uk>
    Signed-off-by: default avatarImre Deak <imre.deak@intel.com>
    Acked-by: default avatarZhao Yakui <yakui.zhao@intel.com>
    Tested-by: default avatarRong R Yang <rong.r.yang@intel.com>
    Reviewed-by: default avatarVille Syrjälä <ville.syrjala@linux.intel.com>
    Link: http://patchwork.freedesktop.org/patch/msgid/1467380406-11954-3-git-send-email-imre.deak@intel.com
    6bee14ed
intel_mocs.c 13.6 KB