linux-kernel - Re: [PATCH v2 2/5] powerpc: module_[32|64].c: replace swap function with built-in one

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Message-ID: <87v9zyoudt.fsf@concordia.ellerman.id.au>
Date:   Mon, 01 Apr 2019 22:08:30 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Andrey Abramov <st5pub@...dex.ru>, vgupta <vgupta@...opsys.com>,
        benh <benh@...nel.crashing.org>, paulus <paulus@...ba.org>,
        tglx <tglx@...utronix.de>, mingo <mingo@...hat.com>,
        bp <bp@...en8.de>, hpa <hpa@...or.com>, x86 <x86@...nel.org>,
        mark <mark@...heh.com>, jlbec <jlbec@...lplan.org>,
        richard <richard@....at>, dedekind1 <dedekind1@...il.com>,
        "adrian.hunter" <adrian.hunter@...el.com>,
        gregkh <gregkh@...uxfoundation.org>,
        "naveen.n.rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        jpoimboe <jpoimboe@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        "darrick.wong" <darrick.wong@...cle.com>,
        "ard.biesheuvel" <ard.biesheuvel@...aro.org>,
        George Spelvin <lkml@....org>,
        linux-snps-arc <linux-snps-arc@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        ocfs2-devel <ocfs2-devel@....oracle.com>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        sfr <sfr@...b.auug.org.au>
Cc:     malat <malat@...ian.org>, npiggin <npiggin@...il.com>,
        "yamada.masahiro" <yamada.masahiro@...ionext.com>
Subject: Re: [PATCH v2 2/5] powerpc: module_[32|64].c: replace swap function with built-in one

Andrey Abramov <st5pub@...dex.ru> writes:
> Replace relaswap with built-in one, because relaswap
> does a simple byte to byte swap.

I replied to v1 with an ack, which stands, this is just a clarification.

> Since Spectre mitigations have made indirect function calls more
> expensive, and the default simple byte copies swap is implemented
> without them, an "optimized" custom swap function is now
> a waste of time as well as code.

Spectre mitigations don't necessarily slow down indirect function calls
on all arches or in all configurations on all arches, so that
justification is a bit dubious.

That's not really a problem for this patch, because the existing
overhead of the function call is probably already enough to rival the
cost of the 24 byte copy.

cheers