Re: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop targets

Andi Kleen
2019-07-16T00:32:25Z

On Mon, Jul 15, 2019 at 04:10:36PM -0700, Andy Lutomirski wrote:
> On Mon, Jul 15, 2019 at 3:53 PM Andi Kleen <ak@linux.intel.com> wrote:
> >
> > > I haven't tested on a real kernel with i915. Does i915 really hit
> > > this code path? Does it happen more than once or twice at boot?
> >
> > Yes some workloads allocate/free a lot of write combined memory
> > for graphics objects.
> >
>
> But where does that memory come from? If it's from device memory
> (i.e. memory that's not in the kernel direct map), then, unless I
> missed something, we're never changing the cache mode per se -- we're
> just ioremap_wc-ing it, which doesn't require a flush.

Integraded graphics doesn't have device memory. There's an reserved
memory area, but a lot of the buffers the GPU works with
come from main memory.

-Andi