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

From: Chris Wilson
Date: Mon Oct 19 2015 - 07:05:56 EST


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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