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

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Date:   Mon, 15 Jul 2019 17:32:22 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Uros Bizjak <ubizjak@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>
Subject: Re: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop targets

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@...ux.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