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Date: Mon, 19 Nov 2018 09:38:55 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jiri Kosina <jkosina@suse.cz>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
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Subject: Re: STIBP by default.. Revert?
Message-ID: <20181119083855.GA129733@gmail.com>
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* Linus Torvalds <torvalds@linux-foundation.org> 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.

Thanks,

	Ingo