From: Andrew Cooper <andrew.cooper3@citrix.com>
To: speck@linutronix.de
Subject: [MODERATED] Re: [PATCH 2/4] RFC-MDSv1 0
Date: Sun, 9 Dec 2018 18:12:52 +0000 [thread overview]
Message-ID: <193b9590-6f56-4b96-6dcc-d003548aa1c7@citrix.com> (raw)
In-Reply-To: <20181114040429.GL6218@tassilo.jf.intel.com>
On 14/11/2018 04:04, speck for Andi Kleen wrote:
>> I was under the impression that the recommendation to use a NUL selector
>> was specifically to avoid this second read.
> No the rationale for the recommendation is to avoid mispredictions
> in the microcode. But the two are equivalent.
After a discussion with Jason where (again) he suggested the use of %ds
rather than NUL, I went back and extended my micro-benchmark.
All other things being equal and cache-hot, a VERW of %ds really is
faster NUL. (Net execution time is about 2/3 as long as recorded by two
rdtscp's, not accounting for any systematic timing errors). This is
just against some of my development boxes - I'm still waiting on the
correct CPU to use the prototype microcode with additional VERW goodies.
Either way - I'm surprised by these results, but %ds does looks to be
the quickest option. Apologies for the noise on this thread.
~Andrew
next prev parent reply other threads:[~2018-12-09 21:08 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-12 3:41 [MODERATED] [PATCH 0/4] RFC-MDSv1 1 Andi Kleen
2018-11-12 3:41 ` [MODERATED] [PATCH 1/4] RFC-MDSv1 4 Andi Kleen
2018-11-12 3:41 ` [MODERATED] [PATCH 2/4] RFC-MDSv1 0 Andi Kleen
2018-11-13 0:07 ` [MODERATED] " Andrew Cooper
2018-11-13 1:35 ` Andi Kleen
2018-11-13 21:59 ` Andrew Cooper
2018-11-14 4:04 ` Andi Kleen
2018-12-09 18:12 ` Andrew Cooper [this message]
2018-11-13 20:47 ` Jiri Kosina
2018-11-13 21:04 ` Luck, Tony
2018-11-14 2:32 ` Andi Kleen
2018-11-12 3:41 ` [MODERATED] [PATCH 3/4] RFC-MDSv1 3 Andi Kleen
2018-11-12 3:41 ` [MODERATED] [PATCH 4/4] RFC-MDSv1 2 Andi Kleen
2018-11-12 22:36 ` [MODERATED] " Greg KH
2018-11-13 1:37 ` Andi Kleen
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