[Mesa-dev] [PATCH] i965: Disable L3 cache allocation for external buffers

[Jason Ekstrand]

On Tue, Oct 24, 2017 at 2:10 PM, Chris Wilson <chris at chris-wilson.co.uk>
wrote:

> Quoting Jason Ekstrand (2017-10-24 22:03:36)
> > On Tue, Oct 24, 2017 at 9:06 AM, Chris Wilson <chris at chris-wilson.co.uk>
> wrote:
> >
> >     Through the use of mocs, we can define the cache usage for any
> surface
> >     used by the GPU. In particular, we can request that L3 cache be
> >     allocated for either a read/write miss so that subsequent reads can
> be
> >     fetched from cache rather than memory. A consequence of this is that
> if
> >     we allocate a L3/LLC cacheline for a read and the object is changed
> in
> >     main memory (e.g. a PCIe write bypassing the CPU) then the next read
> >     will be serviced from the stale cache and not from the new data in
> >     memory. This is an issue for external PRIME buffers where we may miss
> >     the updates entirely if the image is small enough to fit within our
> >     cache.
> >
> >     Currently, we have a single bit to mark all external buffers so use
> that
> >     to tell us when it is unsafe to use a cache override in mocs and
> >     fallback to the PTE value instead (which should be set to the correct
> >     cache level to be coherent amongst all active parties: PRIME,
> scanout and
> >     render). This may be refined in future to limit the override to
> buffers
> >     outside the control of mesa; as buffers being shared between mesa
> >     clients should be able to coordinate themselves without resolves.
> >
> > Why are we throwing away L3 caching here and in gen7 above?  We should
> be able
> > to handle L3 caching with manual texture/render cache
> invalidates/flushes.  Our
> > performance is already going to be not great with linear buffers with no
> LLC,
> > we shouldn't kill it completely.
>
> Because the L3 is shadowing the updates in memory (that bypass the
> ring/system-agent entirely) and not being invalidated. When you then
> read back from the L3/LLC you get stale data.
>

I'm having a bit of trouble parsing that.  Are you saying that the L3 is
trying to pull from the LLC even though we've disabled it through the PTE?
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