kernel-hardening - 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

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