Re: [igt-dev] [PATCH igt] igt/gem_exec_capture: MI_STORE_DWORD requires EXEC_SECURE + DRM_MASTER on ctg/ilk

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>
Cc: igt-dev@lists.freedesktop.org, intel-gfx@lists.freedesktop.org
Subject: Re: [igt-dev] [PATCH igt] igt/gem_exec_capture: MI_STORE_DWORD requires EXEC_SECURE + DRM_MASTER on ctg/ilk
Date: Mon, 12 Feb 2018 20:35:39 +0200	[thread overview]
Message-ID: <20180212183539.GY5453@intel.com> (raw)
In-Reply-To: <151845707613.18923.14072985212441718378@mail.alporthouse.com>

On Mon, Feb 12, 2018 at 05:37:56PM +0000, Chris Wilson wrote:
> Quoting Ville Syrjälä (2018-02-12 17:30:52)
> > On Sat, Feb 10, 2018 at 09:43:38PM +0000, Chris Wilson wrote:
> > > On ctg/ilk, for whatever reason, MI_STORE_DWORD is a privileged operation
> > > so we must request a SECURE batch.
> > 
> > IIRC ctg supposedly introduced some form of ppgtt. Isn't that the
> > reason?
> > 
> > Hmm. Now I wonder how anything works on these platforms. Should the
> > batch itself be executed via ppgtt if it's non-secure? Maybe the hw
> > has a fallback mechanism of some sort to execute via ggtt if ppgtt
> > isn't enabled...
> > 
> > ppgtt enable bit:
> > "When this bit is clear, all memory accesses will be completed using the
> > GGTT. Privileged memory protections will not be enforced (it is
> > acceptable for a non-secure batch buffer to access GGTT space)"
> > 
> > OK. That seems to confirm that part of the theory.
> > 
> > For pre-ctg the spec says:
> > "Although Buffer Security Indicator is implemented, there is no usage
> >  model for it and it need not be validated."
> > 
> > So I'm thinking we should never set the non-secure bit on these old
> > platforms.
> 
> That does open a large can of worms with the ability to write any
> register from userspace or manipulate the pagetables; i.e. requires the
> cmdparser. The usage model has been such that very few commands are
> affected; certainly no one [else] has noticed (afaik).

I suppose. I can't find any notes about this being wrong in the gen2/3
docs. And the gen4 quote just says "need not be validated" after all so
I suppose it doesn't necessarily mean it wasn't validated. And I guess
we should have found out long ago if it's hopelessly broken.

Patch is
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

-- 
Ville Syrjälä
Intel OTC
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