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Date: Wed, 10 Jan 2018 17:47:47 -0800
From: Andi Kleen <andi@firstfloor.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Brian Gerst <brgerst@gmail.com>, Andi Kleen <andi@firstfloor.org>,
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Subject: Re: [PATCH v1 1/8] x86/entry/clearregs: Remove partial stack frame
 in fast system call
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> What I mean is: this whole series is almost certainly a performance regression, it has no off switch, and is doesn't obviously solve any problem.  It' didn't qualify as a so.  And no one has benchmarked it. I think we should seriously consider just not applying it.

Well it's kernel hardening to guard against possible future speculation
attacks. Linus discussed it here for example:

https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1580667.html

For the on/off switch I can add a CONFIG to enable it, even though
it seems somewhat silly.

-Andi