Re: [PATCH v2] drm/i915/tgl: Add sysfs interface to control class-of-service

["Kumar Valsan, Prathap" <prathap.kumar.valsan@intel.com>]

On Mon, Aug 26, 2019 at 10:17:55AM +0100, Chris Wilson wrote:
> Quoting Prathap Kumar Valsan (2019-08-26 00:35:27)
> > To provide shared last-level-cache isolation to cpu workloads running
> > concurrently with gpu workloads, the gpu allocation of cache lines needs
> > to be restricted to certain ways. Currently GPU hardware supports four
> > class-of-service(CLOS) levels and there is an associated way-mask for
> > each CLOS.
> > 
> > Hardware supports reading supported way-mask configuration for GPU using
> > a bios pcode interface. The supported way-masks and the one currently
> > active is communicated to userspace via a sysfs file--closctrl. Admin user
> > can then select a new mask by writing the mask value to the file.
> 
> What impact does this have on inflight work? Do you need to drain the
> submission queue, change the global registers, force an invalidation and
> then restart? Can it be done from inside the GPU so that it is
> serialised with on-going submission?
I believe this should not be impacting the inflight work. Because, way mask
only influnece a new cache allocation on cache miss. Cache hits are not
restricted by way-mask. So even the way-mask is changed, in-flight
requests previously allocated cache lines from other ways will still be
a cache hit until thrashed.

We want to support this on Gen11 as well, where these registers
are context saved and restored and we prime the register values of new contexts
from recorded defaults. What could be the correct way to handle this, write to the
default object or should ask GPU to re-record after modifying the
registers.

Thanks,
Prathap
> -Chris
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