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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
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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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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