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From:   Andi Kleen <ak@linux.intel.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jiri Kosina <jikos@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Casey Schaufler <casey.schaufler@intel.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        "the arch\/x86 maintainers" <x86@kernel.org>,
        stable@vger.kernel.org
Subject: Re: STIBP by default.. Revert?
References: <CAHk-=wg-9FUGU=grF4gKDq1sm1m39Jbs3A_iyLbSSntU47ncwg@mail.gmail.com>
        <nycvar.YFH.7.76.1811182222230.21108@cbobk.fhfr.pm>
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        <CAHk-=wjo9==zLdUuZkCgMx1YBUTpZkD3NnLBPDf7Fw388z5Y6g@mail.gmail.com>
Date:   Sun, 18 Nov 2018 15:56:10 -0800
In-Reply-To: <CAHk-=wjo9==zLdUuZkCgMx1YBUTpZkD3NnLBPDf7Fw388z5Y6g@mail.gmail.com>
        (Linus Torvalds's message of "Sun, 18 Nov 2018 14:36:09 -0800")
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Sun, Nov 18, 2018 at 2:17 PM Jiri Kosina <jikos@kernel.org> wrote:
>> Which gets us back to Tim's fixup patch. Do you still prefer the revert,
>> given the existence of that?
>
> I don't think the code needs to be reverted, but the *behavior* of
> just unconditionally enabling STIBP needs to be reverted.

Actually I think it should be reverted. Yes of course opt-in
is needed.

But also when you opt-in it doesn't make sense to set STIBP
when the sibling is running the same security context, which
is actually a common case. So to even use it properly you would
need some scheduler support to detect these cases and only
enable it then with opt-in. These patches didn't even try to tackle
this problem.

-Andi