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

From: Eero Tamminen <eero.t.tamminen@intel.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Michael T Frederick <michael.t.frederick@intel.com>,
	Valtteri Rantala <valtteri.rantala@intel.com>,
	intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH v2 2/2] drm/i915/bxt: Fix inadvertent CPU snooping due to incorrect MOCS config
Date: Tue, 26 Apr 2016 20:18:33 +0300	[thread overview]
Message-ID: <571FA2E9.10307@intel.com> (raw)
In-Reply-To: <20160426143005.GT8291@phenom.ffwll.local>

Hi,

On 26.04.2016 17:30, Daniel Vetter wrote:
> On Tue, Apr 26, 2016 at 05:26:43PM +0300, Eero Tamminen wrote:
[...]
>> 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,

What do you think it should do with that information?

I assume you to mean, that Mesa needs to know the *amount* of LLC and 
change its behavior based on that amount, not just whether it's present.

In that case Mesa does, and has always totally "sucked".  Mesa on 
earlier GEN(s) cached everything that can be cached, and I assume it to 
try to do that with GEN9 too.

However, based on our MOCS testing on BDW, that actually gives the best 
overall perf results.  On average it doesn't give much, but it was 
better than any straightforward (buffer size/type) heuristics for making 
something not to be cached in effort to utilize LLC "better".

It seemed that LLC is too small to have meaningful generic heuristics 
for normal 3D workloads (or they need to be very complex, something 
needing months of testing & iteration, or be per application, not generic).

eLLC could be a different matter as it's large enough that one can put 
e.g. color/depth buffer there.

Skip cache setting for LLC may also be useful, if it works (as it in a 
sense extends the cache size), and render compression can also change 
things.  Problem with RBC is that it makes assumptions about memory 
areas usage even less reliable as you don't know how well the content 
compresses.

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

	- Eero

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