Re: [PATCH 2/2] drm/i915: Prevent loading of uninitialized context garbage

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Aug 21, 2013 at 04:43:33PM +0300, Ville Syrjälä wrote:
> On Thu, Aug 08, 2013 at 08:00:26PM +0100, Chris Wilson wrote:
> > The extended state bits are stored in the LCA register and affect all
> > updates to the LCA register - i.e. the state on the old context is saved
> > when SAVE_EX_STATE_EN  is currently set in the old context address before
> > the update, and the new context is restored when RESTORE_EX_STATE_EN is
> > set in the new context address. This is irrespective of the
> > RESTORE_INHIBIT flag in the MI_SET_CONTEXT.
> > 
> > Hence, upon initial loading the contents of the extended state is read
> > from uninitialised data. To workaround this, on first load we do a dummy
> > load without the mandatory RESTORE_EX_STATE_EN bit so that the real load
> > causes us to initialise the extended state of the context before it is
> > then loaded by the LCA update.
> > 
> > v2: Split out the introduction of the variable length MI_SET_CONTEXT
> > command sequence.
> > 
> > References: https://bugs.freedesktop.org/show_bug.cgi?id=64073
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Ben Widawsky <ben@bwidawsk.net>
> > ---
> >  drivers/gpu/drm/i915/i915_gem_context.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> > index 8a7b61e..a57d49a 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> > @@ -367,6 +367,8 @@ mi_set_context(struct intel_ring_buffer *ring,
> >  	case 5: len += 2;
> >  		break;
> >  	}
> > +	if (!new_context->is_initialized)
> > +		len += 2;
> >  
> >  	ret = intel_ring_begin(ring, len);
> >  	if (ret)
> > @@ -382,6 +384,22 @@ mi_set_context(struct intel_ring_buffer *ring,
> >  		break;
> >  	}
> >  
> > +	if (!new_context->is_initialized) {
> > +		/* The GPU tries to restore the extended state irrespective
> > +		 * of RestoreInhibit (since it is part of the LCA switch
> > +		 * itself rather than the MI_SET_CONTEXT command).
> > +		 * Since the initial contents may be garbage we do a dummy
> > +		 * load first then set the mandatory flag for any future
> > +		 * ring context switches.
> > +		 */
> > +		intel_ring_emit(ring, MI_SET_CONTEXT);
> > +		intel_ring_emit(ring,
> > +				i915_gem_obj_ggtt_offset(new_context->obj) |
> > +				MI_MM_SPACE_GTT |
> > +				MI_SAVE_EXT_STATE_EN |
> > +				hw_flags);
> > +	}
> 
> Hmm. Couldn't we just do this w/ one MI_SET_CONTEXT? Just drop the
> MI_RESTORE_EXT_STATE_EN flag if the context is not initialized. The
> MI_SAVE_EXT_STATE_EN will be saved in the CCID, so when we switch to
> another context the extended state will be saved. And for the next
> switch to this context we will set the MI_RESTORE_EXT_STATE_EN bit
> in MI_SET_CONTEXT so it should get restored.
> 
> But I must admit BSpec is a bit confusing on the topic. It says the
> restore bit affects the switch to the context specified in the
> logical context address. I take that to mean that the effect of the
> restore bit is immediate. But BSpec also says that the bit is stored in
> CCID to control the subsequent switch to the same context. So does that
> actually mean that 'effective.restore_ext = CCID.restore_ext |
> MI_SET_CONTEXT.restore_ext'?
> 
> Oh, but BSpec also says that both bits must be set when RS2 power state
> is enabled. I think that's the same as RC6, or is it? So I guess the
> hardware might consult these bits when entering/leaving RC6. So I suppose
> we really need to make sure both bits are always set in case we hit RC6.
> So based on that reasoning the patch would seem correct.
> 
> I guess I'll give it an r-b regardless :)
> 
> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

I just noticed that on HSW these bits control the resource streamer
state save/restore. The spec says we should always set the RS
restore bit if we set the RS save bit. So maybe we need some
!IS_HASWELL checks in there...

> 
> > +
> >  	intel_ring_emit(ring, MI_NOOP);
> >  	intel_ring_emit(ring, MI_SET_CONTEXT);
> >  	intel_ring_emit(ring, i915_gem_obj_ggtt_offset(new_context->obj) |
> > -- 
> > 1.8.4.rc1
> > 
> > _______________________________________________
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> > Intel-gfx@lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Ville Syrjälä
> Intel OTC

-- 
Ville Syrjälä
Intel OTC