linux-kernel - Re: [PATCH] x86: Add an explicit barrier() to clflushopt()

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Date:	Mon, 19 Oct 2015 12:05:29 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Borislav Petkov <bp@...en8.de>
Cc:	linux-kernel@...r.kernel.org,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"H . Peter Anvin" <hpa@...ux.intel.com>,
	Imre Deak <imre.deak@...el.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] x86: Add an explicit barrier() to clflushopt()

On Mon, Oct 19, 2015 at 12:16:12PM +0200, Borislav Petkov wrote:
> On Mon, Oct 19, 2015 at 10:58:55AM +0100, Chris Wilson wrote:
> > Adding a barrier() into clflushopt() is enough for GCC to dtrt, but
> > solving why GCC is not seeing the constraints from the alternative_io()
> > would be smarter...
> 
> Hmm, would something like adding the memory clobber to the
> alternative_io() definition work?
> 
> ---
> diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
> index 7bfc85bbb8ff..d923e5dacdb1 100644
> --- a/arch/x86/include/asm/alternative.h
> +++ b/arch/x86/include/asm/alternative.h
> @@ -207,7 +207,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
>  /* Like alternative_input, but with a single output argument */
>  #define alternative_io(oldinstr, newinstr, feature, output, input...)	\
>  	asm volatile (ALTERNATIVE(oldinstr, newinstr, feature)		\
> -		: output : "i" (0), ## input)
> +		: output : "i" (0), ## input : "memory")
>  
>  /* Like alternative_io, but for replacing a direct call with another one. */
>  #define alternative_call(oldfunc, newfunc, feature, output, input...)	\

In order to add the clobbers, I had to adjust the macro slightly:

+#define alternative_output(oldinstr, newinstr, feature, output)        \
+       asm volatile (ALTERNATIVE(oldinstr, newinstr, feature)          \
+               : output : "i" (0) : "memory")

and that also works. The concern I have here is that the asm does use
"+m" as its output already, and the clflush() asm doesn't require the
manual clobber.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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