Re: pin OABUFFER to GGTT

["Mateo Lozano, Oscar" <oscar.mateo@intel.com>]

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> -----Original Message-----
> From: Chris Wilson [mailto:chris@chris-wilson.co.uk]
> Sent: Wednesday, July 02, 2014 7:56 AM
> To: Ben Widawsky
> Cc: Mateo Lozano, Oscar; Intel-gfx@lists.freedesktop.org; Madajczak,
> Tomasz; Rutkowski, Adam J; Jesse Barnes (jbarnes@virtuousgeek.org)
> Subject: Re: [Intel-gfx] pin OABUFFER to GGTT
> 
> On Tue, Jul 01, 2014 at 11:40:52PM -0700, Ben Widawsky wrote:
> > On Tue, Jul 01, 2014 at 08:54:27PM +0100, Chris Wilson wrote:
> > > On Tue, Jul 01, 2014 at 05:16:30PM +0000, Mateo Lozano, Oscar wrote:
> > > > > The issue is they need:
> > > > >
> > > > > A) A buffer object.
> > > > > B) Bound to GGTT.
> > > > > C) That userspace knows the GGTT offset of, so that they can
> > > > > program OABUFFER with it.
> > > > > D) That userspace can map so that they can read the reported
> counters.
> > > > >
> > > > > They used to create a bo, call bo_pin on it, use args->offset to
> > > > > program OABUFFER (via MI_LOAD_REGISTER_IMM, I imagine), map it
> > > > > and read the counter values. They cannot do this anymore.
> > > >
> > > > The answer might be that all of this needs to be done by the kernel
> itself, but then we need to provide an interface to userspace...
> > >
> > > Yes. If you need to pin a buffer for a register, then it needs to be
> > > handled by the kernel. Especially one that provides information
> > > about other users.
> > > -Chris
> > >
> >
> > I'm unclear why they need the offset. Can it not work like any other
> > relocation, except with the requirement that it's in the GGTT?
> >
> > I'd also argue that they need to be able to map it (they need the
> > contents, which may or may not be mapped). However, I think this is a
> > very minor point.
> >
> > With the command validator this should be a pretty reasonable thing to
> > accomplish. I think we can just give a flag for the reloc that it
> > needs to be in the GGTT, and then pass a check to the command scanner
> > that only that one offset is allowed, and only for OA.
> 
> If the intention was to only measure within a batch and the command parser
> ensured that the register was cleared before the end of the batch, fine.
> That's not an information leak nor do we keep the hardware pointing to an
> unpinned buffer afterwards.
> Chris Wilson, Intel Open Source Technology Centre

Tomasz: is the intention to only measure within a batch? My impression is that you wanted to maintain the OABUFFER programmed and then collect performance reports for a period of time (probably for several batchbuffers). If that´s the case, the relocation approach is not possible.

Chris: please notice that the bo_pin ioctl they were using until now required root, so the information leak about other users was not that bad. Still, I tend to agree that bo_pin for this is overkill and the programming of OABUFFER should be carried out by the kernel (with some interface to userspace so that they can retrieve the actual contents of the buffer... root only of course). 

-- Oscar