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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 18 Nov 2018 12:36:08 -0800
Message-ID: <CAHk-=wg-9FUGU=grF4gKDq1sm1m39Jbs3A_iyLbSSntU47ncwg@mail.gmail.com>
Subject: STIBP by default.. Revert?
To:     Jiri Kosina <jkosina@suse.cz>,
        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>
Cc:     Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>, stable@vger.kernel.org
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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.

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".

                  Linus