linux-kernel - Re: [PATCH v5 04/12] x86: introduce __uaccess_begin

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Date:   Tue, 30 Jan 2018 07:56:53 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Greg KH <gregkh@...uxfoundation.org>, X86 ML <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Cox <alan@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 04/12] x86: introduce __uaccess_begin_nospec and ifence


* Dan Williams <dan.j.williams@...el.com> wrote:

> > The flip side is that if the MFENCE stalls the STAC that is ahead of it could be
> > processed for 'free' - while it's always post barrier with my suggestion.
> 
> This 'for free' aspect is what I aiming for.

Ok.

> >
> > But in any case it would be nice to see a discussion of this aspect in the
> > changelog, even if the patch does not change.
> 
> I'll add a note to the changelog that having the fence after the
> 'stac' hopefully allows some overlap of the cost of 'stac' and the
> flushing of the instruction pipeline.

Perfect!

Thanks,

	Ingo