linux-kernel - Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation

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Date:   Thu, 16 Jan 2020 20:57:03 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, nathanl@...ux.ibm.com,
        Arnd Bergmann <arnd@...db.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list\:MIPS" <linux-mips@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation

Andy Lutomirski <luto@...nel.org> writes:
> On Thu, Jan 16, 2020 at 9:58 AM Christophe Leroy
>
> Would mul_u64_u64_shr() be a good alternative?  Could we adjust it to
> assume the shift is less than 32?  That function exists to benefit
> 32-bit arches.

We'd want mul_u64_u32_shr() for this. The rules for mult and shift are:

     1 <= mult <= U32_MAX

     1 <= shift <= 32

If we want to enforce a shift < 32 we need to limit that conditionally
in the calculation/registration function.

Thanks,

        tglx