linux-kernel - Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from

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Date:   Tue, 14 Jan 2020 13:49:03 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user

On Tue, Jan 14, 2020 at 1:37 PM Vineet Gupta <Vineet.Gupta1@...opsys.com> wrote:
>
> On 1/14/20 12:42 PM, Arnd Bergmann wrote:
> >
> > What's wrong with the generic version on little-endian? Any
> > chance you can find a way to make it work as well for you as
> > this copy?
>
> find_zero() by default doesn't use pop count instructions.

Don't you think the generic find_zero() is likely just as fast as the
pop count instruction? On 32-bit, I think it's like a shift and a mask
and a couple of additions.

The 64-bit case has a multiply that is likely expensive unless you
have a good multiplication unit (but what 64-bit architecture
doesn't?), but the generic 32-bit LE code should already be pretty
close to optimal, and it might not be worth it to worry about it.

(The big-endian case is very different, and architectures really can
do much better. But LE allows for bit tricks using the carry chain)

             Linus