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).
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