Re: [PATCH 02/21] drm/i915/gtt: Workaround for HW preload not flushing pdps

Dave Gordon
2015-06-11T07:31:07Z

On 10/06/15 12:42, Michel Thierry wrote:
> On 5/29/2015 1:53 PM, Michel Thierry wrote:
>> On 5/29/2015 12:05 PM, Michel Thierry wrote:
>>> On 5/22/2015 6:04 PM, Mika Kuoppala wrote:
>>>> With BDW/SKL and 32bit addressing mode only, the hardware preloads
>>>> pdps. However the TLB invalidation only has effect on levels below
>>>> the pdps. This means that if pdps change, hw might access with
>>>> stale pdp entry.
>>>>
>>>> To combat this problem, preallocate the top pdps so that hw sees
>>>> them as immutable for each context.
>>>>
>>>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>>> Cc: Rafael Barbalho <rafael.barbalho@intel.com>
>>>> Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
>>>> ---
>>>> drivers/gpu/drm/i915/i915_gem_gtt.c | 50
>>>> +++++++++++++++++++++++++++++++++++++
>>>> drivers/gpu/drm/i915/i915_reg.h | 17 +++++++++++++
>>>> drivers/gpu/drm/i915/intel_lrc.c | 15 +----------
>>>> 3 files changed, 68 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>> b/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>> index 0ffd459..1a5ad4c 100644
>>>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>> @@ -941,6 +941,48 @@ err_out:
>>>> return ret;
>>>> }
>>>>
>>>> +/* With some architectures and 32bit legacy mode, hardware pre-loads
>>>> the
>>>> + * top level pdps but the tlb invalidation only invalidates the
>>>> lower levels.
>>>> + * This might lead to hw fetching with stale pdp entries if top level
>>>> + * structure changes, ie va space grows with dynamic page tables.
>>>> + */

Is this still necessary if we reload PDPs via LRI instructions whenever
the address map has changed? That always (AFAICT) causes sufficient
invalidation, so then we might not need to preallocate at all :)

.Dave.