linux-kernel - Re: [PATCH 06/18] x86, barrier: stop speculation for failed access

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Date:   Mon, 8 Jan 2018 10:57:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        netdev@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

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).