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

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Message-ID: <96394DFF-43D7-4877-AA47-FC847FD26184@zytor.com>
Date: Thu, 29 Feb 2024 17:01:43 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: David Laight <David.Laight@...LAB.COM>,
        "'Thorsten Blum'" <thorsten.blum@...lux.com>,
        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>
CC: "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()

>> 
>> Change deltapm to unsigned long and replace do_div() with div64_ul()
>> which doesn't implicitly cast the divisor and doesn't unnecessarily
>> calculate the remainder.
>
>Eh? they are entirely different beasts.
>
>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.
>
>The remainder is pretty much free in both cases.
>If a cpu has a divide instruction it will almost certainly
>put the result in one register and the quotient in another.
>

Not on e.g. RISC-V.