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

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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