From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok
Date: Mon, 8 Jan 2018 10:57:02 +0100
Message-ID: <20180108095702.GE3040@hirez.programming.kicks-ass.net>
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Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Andi Kleen <ak@linux.intel.com>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        netdev@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
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On Sun, Jan 07, 2018 at 06:57:35PM -0800, Alexei Starovoitov wrote:
> On Sun, Jan 07, 2018 at 01:59:35PM +0000, Alan Cox wrote:

> > lfence timing is also heavily dependent upon what work has to be done to
> > retire previous live instructions. 
> > BPF does not normally do a lot of writing so you'd expect the cost to be low.
> 
> right. to retire previous loads. Not sure what 'not a lot of writing'
> has to do with lfence.

LFENCE will wait for completion on _ALL_ prior instructions, not just
loads.

Stores are by far the most expensive instructions to wait for, as they
only complete once their value is globally visible (on x86).