[Intel-gfx] [Mesa-dev] [PATCH] i965: Revert absolute mode for constant buffer pointers.

[Kenneth Graunke]

On Monday, October 23, 2017 3:53:15 PM PDT Rodrigo Vivi wrote:
> On Mon, Oct 23, 2017 at 10:32:43PM +0000, Jordan Justen wrote:
> > On 2017-10-19 16:30:44, Kristian Høgsberg wrote:
> > > On Thu, Oct 19, 2017 at 4:18 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> > > > The kernel doesn't initialize the value of the INSTPM or CS_DEBUG_MODE2
> > > > registers at context initialization time.  Instead, they're inherited
> > > > from whatever happened to be running on the GPU prior to first run of a
> > > > new context.  So, when we started setting these, other contexts in the
> > > > system started inheriting our values.  Since this controls whether
> > > > 3DSTATE_CONSTANT_* takes a pointer or an offset, getting the wrong
> > > > setting is fatal for almost any process which isn't expecting this.
> > > >
> > > > Unfortunately, VA-API and Beignet don't initialize this (nor does older
> > > > Mesa), so they will die horribly if we start doing this.  UXA and SNA
> > > > don't use any push constants, so they are unaffected.
> > > >
> > > > The kernel developers are apparently uninterested in making the proto-
> > > > context initialize these registers to guarantee deterministic behavior.
> > > 
> > > Could somebody from the kernel team elaborate here? This is obviously
> > > broken and undermines the security and containerization that hw
> > > contexts are supposed to provide. I'm really curious what the thinking
> > > is here...
> > > 
> > > Kristian
> > 
> > Cc intel-gfx, maintainers
> 
> Is this the null-state/golden-context related discussions?
> 
> I assume we are ok for older platforms, but the problem would be only for
> CNL+ where we are not adding the null context initialization yet.
> Am I getting it right?

No, this problem exists on earlier platforms as well.  We saw the issue
on Broadwell and Kabylake.
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