From: Linus Torvalds <torvalds@linux-foundation.org>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: linux-arch <linux-arch@vger.kernel.org>,
Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
Peter Zijlstra <peterz@infradead.org>,
Andrey Konovalov <andreyknvl@google.com>,
Aleksa Sarai <cyphar@cyphar.com>, Ingo Molnar <mingo@kernel.org>,
Khalid Aziz <khalid.aziz@oracle.com>,
Christian Brauner <christian.brauner@ubuntu.com>,
"open list:SYNOPSYS ARC ARCHITECTURE"
<linux-snps-arc@lists.infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user
Date: Tue, 14 Jan 2020 13:49:03 -0800 [thread overview]
Message-ID: <CAHk-=wjX-c9YpPhbQ073UPnTvELNQCN49vqK1yY7JGuHSn5-ew@mail.gmail.com> (raw)
In-Reply-To: <3734021d-1756-3a09-6595-14ca58c64bf9@synopsys.com>
On Tue, Jan 14, 2020 at 1:37 PM Vineet Gupta <Vineet.Gupta1@synopsys.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
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next prev parent reply other threads:[~2020-01-14 21:52 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-14 20:08 [RFC 0/4] Switching ARC to optimized generic strncpy_from_user Vineet Gupta
2020-01-14 20:08 ` [RFC 1/4] asm-generic/uaccess: don't define inline functions if noinline lib/* in use Vineet Gupta
2020-01-14 20:57 ` Arnd Bergmann
2020-01-15 23:01 ` Vineet Gupta
2020-01-16 11:43 ` Arnd Bergmann
2020-01-14 21:32 ` Linus Torvalds
2020-01-15 9:08 ` Arnd Bergmann
2020-01-15 14:12 ` Al Viro
2020-01-15 14:21 ` Arnd Bergmann
2020-01-14 20:08 ` [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check Vineet Gupta
2020-01-14 21:22 ` Linus Torvalds
2020-01-14 21:52 ` Vineet Gupta
2020-01-14 23:46 ` Al Viro
2020-01-15 14:42 ` Andrey Konovalov
2020-01-15 23:00 ` Vineet Gupta
2020-01-14 20:08 ` [RFC 3/4] ARC: uaccess: remove noinline variants of __strncpy_from_user() and friends Vineet Gupta
2020-01-14 20:08 ` [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user Vineet Gupta
2020-01-14 20:42 ` Arnd Bergmann
2020-01-14 21:36 ` Vineet Gupta
2020-01-14 21:49 ` Linus Torvalds [this message]
2020-01-14 22:14 ` Vineet Gupta
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