linux-kernel - Re: [PATCH] x86/apic: Use div64_ul() instead of do

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Date: Fri, 1 Mar 2024 13:28:00 +0100
From: Thorsten Blum <thorsten.blum@...lux.com>
To: David Laight <David.Laight@...LAB.COM>
Cc: Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 "x86@...nel.org" <x86@...nel.org>,
 "H. Peter Anvin" <hpa@...or.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Wei Liu <wei.liu@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/apic: Use div64_ul() instead of do_div()

> On Feb 29, 2024, at 23:13, David Laight <David.Laight@...LAB.COM> wrote:
> 
> do_div() does a 64 by 32 divide that gives a 32bit quotient.
> div64_ul() does a much more expensive 64 by 64 divide that
> can generate a 64bit quotient.

Since the dividend and the divisor could (according to the types at least) both
be 64-bit values and do_div() does a 64-by-32 division, the quotient could
potentially be wrong.

However, if the values don't require a 64-by-64 division (not even on a 64-bit
system) and the divisor (deltapm) is guaranteed to fit into 32 bits, wouldn't it
make sense to change its type from long to int?

Given that acpi_pm_read_early() returns a u32 and is then assigned to unsigned
long pm, this might be the source of the issue. Changing pm and deltapm from
long to u32 and keeping do_div() as is, would improve the types and remove the
Coccinelle warnings...but maybe I'm missing something?

Thanks,
Thorsten