[Intel-gfx] [PATCH] drm/i915/gt: Flush gen3 relocs harder, again

[Chris Wilson]

Quoting Mika Kuoppala (2020-06-13 09:44:39)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > Quoting Mika Kuoppala (2020-06-12 23:05:18)
> >> Chris Wilson <chris at chris-wilson.co.uk> writes:
> >> 
> >> > gen3 does not fully flush MI stores to memory on MI_FLUSH, such that a
> >> > subsequent read from e.g. the sampler can bypass the store and read the
> >> > stale value from memory. This is a serious issue when we are using MI
> >> > stores to rewrite the batches for relocation, as it means that the batch
> >> > is reading from random user/kernel memory. While it is particularly
> >> > sensitive [and detectable] for relocations, reading stale data at any
> >> > time is a worry.
> >> >
> >> > Having started with a small number of delaying stores and doubling until
> >> > no more incoherency was seen over a few hours (with and without
> >> > background memory pressure), 32 was the magic number.
> >> >
> >> > v2: Follow more closer with the gen5 w/a and include some
> >> > post-invalidate flushes as well.
> >> >
> >> > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2018
> >> > References: a889580c087a ("drm/i915: Flush GPU relocs harder for gen3")
> >> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> >> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> >> > ---
> >> >  drivers/gpu/drm/i915/gt/gen2_engine_cs.c | 61 ++++++++++--------------
> >> >  1 file changed, 25 insertions(+), 36 deletions(-)
> >> >
> >> > diff --git a/drivers/gpu/drm/i915/gt/gen2_engine_cs.c b/drivers/gpu/drm/i915/gt/gen2_engine_cs.c
> >> > index 3fb0dc1fb910..5400d657f334 100644
> >> > --- a/drivers/gpu/drm/i915/gt/gen2_engine_cs.c
> >> > +++ b/drivers/gpu/drm/i915/gt/gen2_engine_cs.c
> >> > @@ -13,28 +13,25 @@
> >> >  
> >> >  int gen2_emit_flush(struct i915_request *rq, u32 mode)
> >> >  {
> >> > -     unsigned int num_store_dw;
> >> > +     unsigned int num_store_dw = 12;
> >> >       u32 cmd, *cs;
> >> >  
> >> >       cmd = MI_FLUSH;
> >> > -     num_store_dw = 0;
> >> >       if (mode & EMIT_INVALIDATE)
> >> >               cmd |= MI_READ_FLUSH;
> >> > -     if (mode & EMIT_FLUSH)
> >> > -             num_store_dw = 4;
> >> >  
> >> > -     cs = intel_ring_begin(rq, 2 + 3 * num_store_dw);
> >> > +     cs = intel_ring_begin(rq, 2 + 4 * num_store_dw);
> >> >       if (IS_ERR(cs))
> >> >               return PTR_ERR(cs);
> >> >  
> >> >       *cs++ = cmd;
> >> >       while (num_store_dw--) {
> >> > -             *cs++ = MI_STORE_DWORD_IMM | MI_MEM_VIRTUAL;
> >> > -             *cs++ = intel_gt_scratch_offset(rq->engine->gt,
> >> > -                                             INTEL_GT_SCRATCH_FIELD_DEFAULT);
> >> > -             *cs++ = 0;
> >> > +             *cs++ = MI_STORE_DWORD_INDEX;
> >> > +             *cs++ = I915_GEM_HWS_SCRATCH * sizeof(u32);
> >> > +             *cs++ = rq->fence.seqno - 1;
> >> > +             *cs++ = MI_FLUSH | MI_NO_WRITE_FLUSH;
> >> >       }
> >> > -     *cs++ = MI_FLUSH | MI_NO_WRITE_FLUSH;
> >> > +     *cs++ = cmd;
> >> >  
> >> >       intel_ring_advance(rq, cs);
> >> >  
> >> > @@ -142,38 +139,21 @@ int gen4_emit_flush_vcs(struct i915_request *rq, u32 mode)
> >> >       return 0;
> >> >  }
> >> >  
> >> > -u32 *gen3_emit_breadcrumb(struct i915_request *rq, u32 *cs)
> >> > +static u32 *__gen2_emit_breadcrumb(struct i915_request *rq, u32 *cs,
> >> > +                                int flush, int post)
> >> >  {
> >> >       GEM_BUG_ON(i915_request_active_timeline(rq)->hwsp_ggtt != rq->engine->status_page.vma);
> >> >       GEM_BUG_ON(offset_in_page(i915_request_active_timeline(rq)->hwsp_offset) != I915_GEM_HWS_SEQNO_ADDR);
> >> >  
> >> >       *cs++ = MI_FLUSH;
> >> >  
> >> > -     *cs++ = MI_STORE_DWORD_INDEX;
> >> > -     *cs++ = I915_GEM_HWS_SEQNO_ADDR;
> >> > -     *cs++ = rq->fence.seqno;
> >> > -
> >> > -     *cs++ = MI_USER_INTERRUPT;
> >> 
> >> How can you throw the interrupt part out?
> >
> > Diff being confusing. gen3_emit_breadcrumb and gen5_emit_breadcrumb
> > merged together.
> 
> Reviewed-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>

It failed eventually, so unfortunately it's still just paper. However
increasing the MTBF by a few orders of magnitude should be enough to
stop CI complaining on every idle run.
-Chris