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 Fri,  5 Jan 2018 12:30:48 +0000 (UTC)
Subject: Re: A more general possible meltdown/spectre countermeasure
To: =?UTF-8?Q?Dag-Erling_Sm=c3=b8rgrav?= <des@des.no>
Cc: "freebsd-security@freebsd.org" <freebsd-security@freebsd.org>
References: <c98b7ac3-26f0-81ee-2769-432697f876e5@metricspace.net>
 <86efn4u3fv.fsf@desk.des.no>
From: Eric McCorkle <eric@metricspace.net>
Message-ID: <4bad69c4-6fc7-6735-6b15-81baaee358f3@metricspace.net>
Date: Fri, 5 Jan 2018 07:30:48 -0500
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On 01/05/2018 03:15, Dag-Erling Smørgrav wrote:
> Eric McCorkle <eric@metricspace.net> writes:
>> The obvious downside is that you take a performance hit storing things
>> in non-cacheable locations, especially if you plan on doing heavy
>> computation in that memory (say, encryption/decryption).  However, this
>> is almost certainly going to be less than the projected 30-50%
>> performance hit from other mitigations.
> 
> Where did you get those numbers?  Because the worst documented case for
> KPTI is ~20% for I/O-intensive workloads, and PCID is likely to bring
> this down to single digits if used correctly.  The KAISER paper claims a
> slowdown of < 1%, but that may have been the result of undisclosed
> features of the specific CPU they tested on.

Those were numbers being thrown around.  I'm not putting a lot of stake
in them.