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 Tue, Apr 26, 2016 at 04:30:05PM +0200, Daniel Vetter 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?
> > 
> > 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.
> > 
> > If app runs on HW where LLC/eLLC is missing, giving the app extra slowdown
> > instead of potential speedup sounds like failed HW abstraction. :-)
> 
> Well mesa needs to know llc vs. !llc anyway to not totally suck, and
> defining entry #2 as "coherent, always" makes sense. I thought entry 0 was
> the reaonable default aka pte passthrough and hence managed by kernel?

Nope, we fscked that up somewhat, and entry 1 is the PTE one :( So if
userspace forgets to set MOCS on gen9 it won't get the behaviour it
would have gotten on previous gens.

> 
> If mesa asks for nonsense, the kernel is happy to oblige.

We never really defined what entry 2 actually means: coherent or sane
performance. Mesa has, rightfully IMO, made the assumption that it means
the latter since we never set out to define any MOCS entries with
coherency in mind. And seeing that it's already out in the wild, I think
it's better to respect it. If we change it now we would just make it
more painful for people when they get their hands on the hardware.

I think what we should do is define what the MOCS indexes mean in some
uapi header. Then there would be no ambiguity.

-- 
Ville Syrjälä
Intel OTC
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