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

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Message-ID: <20151019101612.GA20341@pd.tnic>
Date:	Mon, 19 Oct 2015 12:16:12 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Chris Wilson <chris@...is-wilson.co.uk>
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 10:58:55AM +0100, Chris Wilson wrote:
> During testing we observed that the last cacheline was not being flushed
> from a
> 
> 	mb()
> 	for (addr = addr & -clflush_size; addr < end; addr += clflush_size)
> 		clflushopt();
> 	mb()
> 
> loop (where the initial addr and end were not cacheline aligned).
> 
> Changing the loop from addr < end to addr <= end, or replacing the
> clflushopt() with clflush() both fixed the testcase. Hinting that GCC
> was miscompling the assembly within the loop and specifically the
> alternative within clflushopt() was confusing the loop optimizer.
> 
> 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...)	\

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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