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

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Date:   Thu, 11 Jul 2019 21:00:06 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop targets

On Thu, Jul 11, 2019 at 4:39 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> On Thu, Jul 11, 2019 at 1:13 AM Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > Recent patch [1] disabled a self-snoop feature on a list of processor
> > models with a known errata, so we are confident that the feature
> > should work on remaining models also for other purposes than to speed
> > up MTRR programming.
> >
> > I would like to resurrect an old patch [2] that avoids calling clflush
> > and wbinvd
> > to invalidate caches when CPU supports selfsnoop.
>
> The big question here is: what are all the reasons that we might need
> to flush?  Certainly, for stuff like SEV and MKTME, we need to flush
> regardless of any self-snoop capability.

No AMD target defines self-snoop capability, and set_memory_encrypted
forces cache clearing in __set_memory_enc_dec:

    /*
     * Before changing the encryption attribute, we need to flush caches.
     */
    cpa_flush(&cpa, 1);

Uros.