Date: Tue, 9 Jan 2018 17:34:33 -0800
From: Andi Kleen <andi@firstfloor.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andi Kleen <andi@firstfloor.org>, tglx@linutronix.de,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, dwmw@amazon.co.uk, pjt@google.com,
        luto@kernel.org, peterz@infradead.org, thomas.lendacky@amd.com,
        tim.c.chen@linux.intel.com, gregkh@linux-foundation.org,
        dave.hansen@intel.com, jikos@kernel.org
Subject: Re: x86/clearregs: Register sanitizing at kernel entry for
 speculation hygiene
Message-ID: <20180110013433.mmw7j3uy7xod5e25@two.firstfloor.org>
References: <20180110010328.22163-1-andi@firstfloor.org>
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> I don't like this at all.  Once upon a time, Linux syscalls were supposed to be fast.  Then we learned about the Meltdown screwup, so we mostly fixed it for real upstream and the distroa seriously half-arsed their own fixes [1].  This came with a big performance cost, but it can be turned off on non-busted hardware.  So be it.

That's true, but modern CPUs are also a lot faster/wider than the K8
the fast path was originally designed for. A modern CPU can go through
these instructions really fast with a very high IPC because they don't have
dependencies or stalls.

So it shouldn't hurt very much.

Also in fact when the fast path was originally written the ABI still had a
different caller/callee split which made it more better. Later on
it already lost some of its benefits and was less of a win.
 
> But now we're proposing to throw out the whole fast path because it might make it a bit harder to do the most obvious attack.  Not very hard, mind you, but a little bit harder.  And there's no off switch for less-leaky hardware.  No thanks.

Well the off switch is a fast CPU.

-Andi