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

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Date:   Tue, 14 Jan 2020 22:14:31 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 1/14/20 1:49 PM, Linus Torvalds wrote:
> 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.

You are right that in grand scheme things it may be less than noise.

ARC pop count version

# 	bits = (bits - 1) & ~bits;
#  	return bits >> 7;

	sub r0,r6,1
	bic r6,r0,r6
	lsr r0,r6,7

# 	return fls(mask) >> 3;

	fls.f	r0, r0
	add.nz	r0, r0, 1
	asr r5,r0,3

	j_s.d [blink]

Generic version

# 	bits = (bits - 1) & ~bits;
#  	return bits >> 7;

	sub r5,r6,1
	bic r6,r5,r6
	lsr r5,r6,7

#  	unsigned long a = (0x0ff0001+mask) >> 23;
# 	return a & mask;

	add r0,r5,0x0ff0001	<-- this is 8 byte instruction though
	lsr_s r0,r0,23
	and r5,r5,r0

	j_s.d [blink]


But its the usual itch/inclination of arch people to try and use the specific
instruction if available.

> 
> 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)

-Vineet