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Date: Sun, 28 Jan 2018 10:14:37 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Dan Williams <dan.j.williams@...el.com>
Cc: tglx@...utronix.de, linux-arch@...r.kernel.org,
Tom Lendacky <thomas.lendacky@....com>,
Andi Kleen <ak@...ux.intel.com>, Kees Cook <keescook@...omium.org>,
kernel-hardening@...ts.openwall.com, gregkh@...uxfoundation.org,
x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
Al Viro <viro@...iv.linux.org.uk>, "H. Peter Anvin" <hpa@...or.com>,
torvalds@...ux-foundation.org, alan@...ux.intel.com,
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:
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -124,6 +124,11 @@ extern int __get_user_bad(void);
>
> #define __uaccess_begin() stac()
> #define __uaccess_end() clac()
> +#define __uaccess_begin_nospec() \
> +({ \
> + stac(); \
> + ifence(); \
> +})
BTW., wouldn't it be better to switch the barrier order here, i.e. to do:
ifence(); \
stac(); \
?
The reason is that stac()/clac() is usually paired, so there's a chance with short
sequences that it would resolve with 'no externally visible changes to flags'.
Also, there's many cases where flags are modified _inside_ the STAC/CLAC section,
so grouping them together inside a speculation atom could be beneficial.
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.
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.
Thanks,
Ingo
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