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

[Chris Wilson]

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.
-Chris