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

[Andy Lutomirski]

On Mon, Jul 15, 2019 at 3:53 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> 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.

IOW I'm wondering if there's any workload where this patch makes a difference.