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Date:   Mon, 19 Nov 2018 09:43:22 +0100 (CET)
From:   Jiri Kosina <jikos@kernel.org>
To:     Ingo Molnar <mingo@kernel.org>
cc:     Linus Torvalds <torvalds@linux-foundation.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>,
        Andi Kleen <ak@linux.intel.com>,
        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?
In-Reply-To: <20181119083855.GA129733@gmail.com>
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References: <CAHk-=wg-9FUGU=grF4gKDq1sm1m39Jbs3A_iyLbSSntU47ncwg@mail.gmail.com> <20181119083855.GA129733@gmail.com>
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On Mon, 19 Nov 2018, Ingo Molnar wrote:

> > This was marked for stable, and honestly, nowhere in the discussion
> > did I see any mention of just *how* bad the performance impact of this
> > was.
> 
> Yeah. This was an oversight - we'll fix it!
> 
> > When performance goes down by 50% on some loads, people need to start
> > asking themselves whether it was worth it. It's apparently better to
> > just disable SMT entirely, which is what security-conscious people do
> > anyway.
> > 
> > So why do that STIBP slow-down by default when the people who *really*
> > care already disabled SMT?
> > 
> > I think we should use the same logic as for L1TF: we default to
> > something that doesn't kill performance. Warn once about it, and let
> > the  crazy people say "I'd rather take a 50% performance hit than
> > worry about a theoretical issue".
> 
> Yeah, absolutely.
> 
> We'll also require performance measurements in changelogs enabling any 
> sort of mitigation feature from now on - this requirement was implicit 
> but 53c613fe6349 flew in under the radar, so it's going to be explicit an 
> explicit requirement.

Do you already have an idea whether you'd proceed with Tim's patchset for 
current cycle? If so, great as far as I am concerned. If not, I'll send a 
patch that switches this to opt-in via kernel cmdline (+boot-time warning 
if not mitigated).

Thanks,

-- 
Jiri Kosina
SUSE Labs