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

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Message-ID: <CALCETrVonxn6tDkxZnbetM9W4Uxxm7-M-tv1e7YsieX3U5OBKA@mail.gmail.com>
Date:   Mon, 15 Jul 2019 15:12:15 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Uros Bizjak <ubizjak@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop targets

On Mon, Jul 15, 2019 at 12:38 PM Andi Kleen <ak@...ux.intel.com> wrote:
>
> >
> > That does not answer the question whether it's worthwhile to do that.
>
> It's likely worthwhile for (Intel integrated) graphics.
>
> There was also a recent issue with 3dxp/dax, which uses ioremap in some
> cases.
>


FWIW, I applied this simpler patch:

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 6a9a77a403c9..a933f99b176a 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -1729,6 +1729,7 @@ static int change_page_attr_set_clr(unsigned
long *addr, int numpages,
         * attributes:
         */
        cache = !!pgprot2cachemode(mask_set);
+       WARN_ON(cache);

        /*
         * On error; flush everything to be sure.

and booted a VM, including loading a module.  The warning did not
fire.  For the most part, we use PAT for things like ioremap_wc(), but
there's no flush, since there's no preexisting mapping at all.

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?

The only case I can think of where this would really matter is DAX, if
anyone uses WT for DAX.