linux-kernel - RE: [PATCH 1/3] x86/tsc: Use div64_ul instead of do

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Date: Thu, 2 May 2024 16:30:18 +0000
From: David Laight <David.Laight@...LAB.COM>
To: "'H. Peter Anvin'" <hpa@...or.com>, Jules Irenge <jbi.octave@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>
CC: "tglx@...utronix.de" <tglx@...utronix.de>, "bp@...en8.de" <bp@...en8.de>,
	"x86@...nel.org" <x86@...nel.org>, "peterz@...radead.org"
	<peterz@...radead.org>, "rdunlap@...radead.org" <rdunlap@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/3] x86/tsc: Use div64_ul instead of do_div()

From: H. Peter Anvin 
> Sent: 28 April 2024 20:13
> 
> On April 28, 2024 10:31:16 AM PDT, Jules Irenge <jbi.octave@...il.com> wrote:
> >do_div() truncates a u64 divisor to 32 bit.
> >This can lead to non-zero being truncated to zero for division.
> >
> >Fix coccinelle warning
> >WARNING: do_div() does a 64-by-32 division, please consider using div64_ul instead
> >
> >Signed-off-by: Jules Irenge <jbi.octave@...il.com>
> >---
> > arch/x86/kernel/tsc.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> >index 5a69a49acc96..2da37f33dd17 100644
> >--- a/arch/x86/kernel/tsc.c
> >+++ b/arch/x86/kernel/tsc.c
> >@@ -484,7 +484,7 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
> >
> > 	/* Calculate the PIT value */
> > 	delta = t2 - t1;
> >-	do_div(delta, ms);
> >+	div64_ul(delta, ms);
> > 	return delta;
> > }
> >
> 
> Are you sure this is not a false positive? This is a *much* more expensive operation on 32 bits.

IIRC do_div() isn't that great either.
Somewhere it changed from being a simple wrapper on the x86 'div' instruction
(ie 64 by 32 giving 32 and 32) to being able to generate a 64bit quotent.

But the patch is entirely broken as well.

	David

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