linux-kernel - Re: [PATCH] x86/math64: Provide a sane mul_u64_u32_div() implementation for x86

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Date:   Thu, 29 Aug 2019 11:30:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com,
        x86@...nel.org
Subject: Re: [PATCH] x86/math64: Provide a sane mul_u64_u32_div()
 implementation for x86_64

On Wed, Aug 28, 2019 at 06:11:23PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 28, 2019 at 05:19:21PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 26, 2019 at 07:47:35AM -0700, kan.liang@...ux.intel.com wrote:
> 
> > > +	return  mul_u64_u32_div(slots, val, 0xff);
> > 
> > But also; x86_64 seems to lack a sane implementation of that function,
> > and it currently compiles into utter crap (it can be 2 instructions).

This one actually builds defconfig :-)

---
Subject: x86/math64: Provide a sane mul_u64_u32_div() implementation for x86_64
From: Peter Zijlstra <peterz@...radead.org>
Date: Wed Aug 28 17:39:46 CEST 2019

On x86_64 we can do a u64 * u64 -> u128 widening multiply followed by
a u128 / u64 -> u64 division to implement a sane version of
mul_u64_u32_div().

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/div64.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
index 20a46150e0a8..9b8cb50768c2 100644
--- a/arch/x86/include/asm/div64.h
+++ b/arch/x86/include/asm/div64.h
@@ -73,6 +73,19 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
 
 #else
 # include <asm-generic/div64.h>
+
+static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 div)
+{
+	u64 q;
+
+	asm ("mulq %2; divq %3" : "=a" (q)
+				: "a" (a), "rm" ((u64)mul), "rm" ((u64)div)
+				: "rdx");
+
+	return q;
+}
+#define mul_u64_u32_div	mul_u64_u32_div
+
 #endif /* CONFIG_X86_32 */
 
 #endif /* _ASM_X86_DIV64_H */