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Date: Thu, 11 Jan 2018 10:44:40 -0800
From: Andi Kleen <andi@firstfloor.org>
To: Andi Kleen <andi@firstfloor.org>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Brian Gerst <brgerst@gmail.com>,
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        Jiri Kosina <jikos@kernel.org>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH v1 1/8] x86/entry/clearregs: Remove partial stack frame
 in fast system call
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> 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.

I did some micro benchmarking now, sampling different real system
calls.

For the entry code (entry to call) I get on average 62 cycles 
for the old code, vs 78 cycles with clear regs and full
stack frame saving on Skylake.

So it's roughly ~20 cycles difference, if we include the restore.

I would conclude 20 cycles are not significant for a syscall,
so there's not a lot of motivation to add a switch
for less security.

-Andi