[Intel-gfx] [PATCH] drm/i915: Disable lazy PPGTT page table optimization for vGPU

[Chris Wilson]

Quoting Joonas Lahtinen (2017-10-18 11:25:43)
> When running under virtualization (vGPU active), we must disable
> the azy PPGTT page table initialization optimization introduced by:
> 
> 14826673247e ("drm/i915: Only initialize partially filled pagetables")
> 
> We must do this because GVT-g makes unduly assumptions about guest
> behaviour, which this optimization breaks. This results in following
> looking errors in the host:
> 
> ERROR gvt: guest page write error -22, gfn 0x7ada8, pa 0x7ada89a8, var 0x6, len 1
> 
> The real fix is to not to depend on i915 driver behaviour, but instead
> either rely on only the contracts that i915 has with the hardware, or
> add some paravirtualization. While the real fix is en route, it won't
> be finished in time for 4.15, so the best option is to disable the
> optimization for now when vGPU is active to avoid breaking 4.15 guests
> in existing VM environments.
> 
> Fixes: 14826673247e ("drm/i915: Only initialize partially filled pagetables")
> Suggested-by: Xiaolin Zhang <xiaolin.zhang at intel.com>
> Signed-off-by: Xiaolin Zhang <xiaolin.zhang at intel.com>
> [Joonas: Rewrote the commit message and added tags.]
> Signed-off-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Zhenyu Wang <zhenyuw at linux.intel.com>
> Cc: Zhi Wang <zhi.a.wang at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Matthew Auld <matthew.auld at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>

Acked-by: Chris Wilson <chris at chris-wilson.co.uk>

I do not claim to understand why, even after reading the explanation.
-Chris