linux-kernel - Re: [PATCH 01/17] bitfield: Add non-constant field

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87sfvm55ct.fsf@codeaurora.org>
Date:   Wed, 24 Nov 2021 10:24:02 +0200
From:   Kalle Valo <kvalo@...eaurora.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Johannes Berg <johannes@...solutions.net>,
        Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Paul Walmsley <paul@...an.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Tero Kristo <kristo@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Lorenzo Bianconi <lorenzo.bianconi83@...il.com>,
        Benoit Parrot <bparrot@...com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Andrew Jeffery <andrew@...id.au>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Joel Stanley <joel@....id.au>,
        Ping-Ke Shih <pkshih@...ltek.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Magnus Damm <magnus.damm@...il.com>,
        Eduardo Valentin <edubezval@...il.com>,
        Keerthy <j-keerthy@...com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        linux-arm-kernel@...ts.infradead.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org, linux-iio@...r.kernel.org,
        linux-media@...r.kernel.org, linux-mmc@...r.kernel.org,
        linux-aspeed@...ts.ozlabs.org, openbmc@...ts.ozlabs.org,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-gpio@...r.kernel.org, linux-pm@...r.kernel.org,
        alsa-devel@...a-project.org
Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}() helpers

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

> Hi Johannes,
>
> On Mon, Nov 22, 2021 at 5:33 PM Johannes Berg <johannes@...solutions.net> wrote:
>> On Mon, 2021-11-22 at 16:53 +0100, Geert Uytterhoeven wrote:
>> > The existing FIELD_{GET,PREP}() macros are limited to compile-time
>> > constants.  However, it is very common to prepare or extract bitfield
>> > elements where the bitfield mask is not a compile-time constant.
>> >
>>
>> I'm not sure it's really a good idea to add a third API here?
>>
>> We have the upper-case (constant) versions, and already
>> {u32,...}_get_bits()/etc.
>
> These don't work for non-const masks.
>
>> Also, you're using __ffs(), which doesn't work for 64-bit on 32-bit
>> architectures (afaict), so that seems a bit awkward.
>
> That's a valid comment. Can be fixed by using a wrapper macro
> that checks if typeof(mask) == u64, and uses an __ffs64() version when
> needed.
>
>> Maybe we can make {u32,...}_get_bits() be doing compile-time only checks
>> if it is indeed a constant? The __field_overflow() usage is already only
>> done if __builtin_constant_p(v), so I guess we can do the same with
>> __bad_mask()?
>
> Are all compilers smart enough to replace the division by
> field_multiplier(field) by a shift?

It looks like the answer is no as few weeks back I received a comment
internally that a team is seeing a slow down with u32_get_bits():

"Time taken for executing both the macros/inline function (in terms of microseconds)
(out of 3 Trails)
FIELD_GET	: 32, 31, 32
u32_get_bits	: 6379, 6664, 6558"

Sadly I didn't realise to ask what compiler they were using. But I still
prefer {u32,...}_get_bits() over FIELD_GET(), they are just so much
cleaner to use.

-- 
https://patchwork.kernel.org/project/linux-wireless/list/

https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches