Re: [PATCH v2 2/2] drm/i915/bxt: Fix inadvertent CPU snooping due to incorrect MOCS config

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

On Thu, Apr 28, 2016 at 04:48:37PM +0200, Daniel Vetter wrote:
> On Thu, Apr 28, 2016 at 11:38:55AM +0300, Imre Deak wrote:
> > On to, 2016-04-28 at 10:17 +0200, Daniel Vetter wrote:
> > > On Tue, Apr 26, 2016 at 07:01:06PM +0300, Imre Deak wrote:
> > > > On ti, 2016-04-26 at 15:42 +0100, Chris Wilson wrote:
> > > > > On Tue, Apr 26, 2016 at 05:26:43PM +0300, Eero Tamminen wrote:
> > > > > > Hi,
> > > > > > 
> > > > > > On 26.04.2016 16:23, Chris Wilson wrote:
> > > > > > > On Tue, Apr 26, 2016 at 04:17:55PM +0300, Imre Deak wrote:
> > > > > > > > On ti, 2016-04-26 at 13:57 +0100, Chris Wilson wrote:
> > > > > > > > > On Tue, Apr 26, 2016 at 03:44:22PM +0300, Imre Deak wrote:
> > > > > > > > > > Setting a write-back cache policy in the MOCS entry
> > > > > > > > > > definition also
> > > > > > > > > > implies snooping, which has a considerable overhead. This
> > > > > > > > > > is
> > > > > > > > > > unexpected for a few reasons:
> > > > > > > > > 
> > > > > > > > > If it is snooping, then I don't see why it is undesirable to
> > > > > > > > > have it
> > > > > > > > > available in a mocs setting. If it is bogus and the bit is
> > > > > > > > > undefined,
> > > > > > > > > then by all means remove it.
> > > > > > > > 
> > > > > > > > None of these entries are used alone for coherent surfaces. For
> > > > > > > > that
> > > > > > > > the application would have to use entry index#1 or #2 _and_
> > > > > > > > call the
> > > > > > > > set caching IOCTL to set the corresponding buffer to be cached.
> > > > > > > 
> > > > > > > No, the application doesn't. There are sufficent interfaces
> > > > > > > exposed that
> > > > > > > userspace can bypass the kernel if it so desired.
> > > > > > > 
> > > > > > > > The
> > > > > > > > problem is that without setting the buffer to be cacheable the
> > > > > > > > expectation is that we won't be snooping and incur the
> > > > > > > > corresponding
> > > > > > > > overhead. This is what this patch addresses.
> > > > > > > 
> > > > > > > Not true.
> > > > > > > 
> > > > > > > > The bit is also bogus, if we wanted snooping via MOCS we'd use
> > > > > > > > the
> > > > > > > > dedicated HW flag for that.
> > > > > > > 
> > > > > > > But you keep saying this bit *enables* snooping. So either it
> > > > > > > does or it
> > > > > > > doesn't.
> > > > > > > 
> > > > > > > > If we wanted to have a snooping MOCS entry we should add that
> > > > > > > > separately (as a forth entry), but we'd need this change as a
> > > > > > > > fix for
> > > > > > > > current users.
> > > > > > > 
> > > > > > > The current users who are getting what they request but don't
> > > > > > > know what
> > > > > > > they were requesting?
> > > > > > 
> > > > > > What this kernel ABI (index entry #2) has been agreed & documented
> > > > > > to provide?
> > > > > 
> > > > > The ABI is what we agree makes sense between hardware / kernel /
> > > > > userspace, and then we stick to it.
> > > > > 
> > > > > > I thought this entry is supposed to replace the writeback LLC/eLLC
> > > > > > cache MOCS setting Mesa is using on (e.g. BDW) to speed up accesses
> > > > > > to a memory area which it knows always to be accessed so that it
> > > > > > can
> > > > > > be cached.
> > > > > 
> > > > > Which it does... Only it speeds us writeback from the CPU, not the
> > > > > GPU. ;)
> > > > > 
> > > > > The confusion seems to be in mistaking !llc for llc. We have to come
> > > > > to
> > > > > some agreement on whether it makes sense having multiple entries for
> > > > > the
> > > > > same follows-PTE mocs or whether it makes more sense to expose the hw
> > > > > capabilties.
> > > > 
> > > > Note that we can't just say to Mesa to use index #0 on BXT instead of
> > > > index #2, since index #0 turns off caching in GPU L3, so we'd have to
> > > > also redefine that to be L3 cached. And I don't know what turning on L3
> > > > caching for index #0 would break, I would rather avoid doing that. Imo
> > > > defining the entries the following way would allow us to use the same
> > > > indexes on GEN9 regardless of it being SKL or BXT:
> > > > #0: uncached (neither in L3 nor in (e)LLC)
> > > > #1: PTE passthrough
> > > 
> > > So our rendercpy in igt does set MOCS entry 1. Or how exactly do all the
> > > set_caching vs. rendercpy tests we currently have pass? Just not?
> > 
> > We don't have tests for coherent surfaces. The current rendercpy test
> > just uses uncached buffers, so they are flushed before checking the
> > result. I could add a new subtest to rendercpy to test with a
> > cached/coherent surface.
> > 
> > > Also, you're guaranateeing that opencl/libva don't screw this up either?
> > 
> > If they don't set the given buffer to be cached via the set_caching
> > IOCTL (as a consequence making them coherent) they are already screwed
> > on CHV. If they call the IOCTL they are fine on BXT too.
> 
> We do implicit set_caching when displaying something to something
> coherent. To make that work userspace should use the "use PTE" mode by
> default, except when they really know what they're doing. That's also the
> mode that's supposed to give you the most reasonable performance. But
> somehow that mode ended up in MOCS entry 1, so pretty much guaranteed
> userspace will get it wrong. Mesa just hit a perf snag, but might as well
> have been visual corruption. I think it'd be a lot safe to make "use PTE"
> entry 0.

If no one so far uses MOCS entry 0, we could redefine it safely. Mesa
doesn't use it (at least on purpose, it might by accident though).
I have no idea about anyone else.

-- 
Ville Syrjälä
Intel OTC
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