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

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Message-ID: <CALCETrW8yTRyP3qnOv04B2XvR5ZHDUky15CCBR2gtNVG3bea-Q@mail.gmail.com>
Date:   Mon, 15 Jul 2019 16:10:36 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 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.

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