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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Mon, 9 Nov 2020 15:41:53 +0100
Message-ID: <CAK8P3a2FMkMc0K+hu0pnqC8wEMeapKPkZXaBm+HFYYPTes5NHA@mail.gmail.com>
Subject: Re: [PATCH v12 4/4] gpio: xilinx: Utilize generic bitmap_get_value
 and _set_value
To:     William Breathitt Gray <vilhelm.gray@gmail.com>
Cc:     Syed Nayyar Waris <syednwaris@gmail.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Michal Simek <michal.simek@xilinx.com>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Mon, Nov 9, 2020 at 2:41 PM William Breathitt Gray
<vilhelm.gray@gmail.com> wrote:
> On Mon, Nov 09, 2020 at 06:04:11PM +0530, Syed Nayyar Waris wrote:
>
> One of my concerns is that we're incurring the latency two additional
> conditional checks just to suppress a compiler warning about a case that
> wouldn't occur in the actual use of bitmap_set_value(). I'm hoping
> there's a way for us to suppress these warnings without adding onto the
> latency of this function; given that bitmap_set_value() is intended to
> be used in loops, conditionals here could significantly increase latency
> in drivers.

At least for this caller, the size check would be a compile-time
constant that can be eliminated.

> I wonder if array_index_nospec() might have the side effect of
> suppressing these warnings for us. For example, would this work:
>
> static inline void bitmap_set_value(unsigned long *map,
>                                     unsigned long value,
>                                     unsigned long start, unsigned long nbits)
> {
>         const unsigned long offset = start % BITS_PER_LONG;
>         const unsigned long ceiling = round_up(start + 1, BITS_PER_LONG);
>         const unsigned long space = ceiling - start;
>         size_t index = BIT_WORD(start);
>
>         value &= GENMASK(nbits - 1, 0);
>
>         if (space >= nbits) {
>                 index = array_index_nospec(index, index + 1);
>
>                 map[index] &= ~(GENMASK(nbits - 1, 0) << offset);
>                 map[index] |= value << offset;
>         } else {
>                 index = array_index_nospec(index, index + 2);
>
>                 map[index + 0] &= ~BITMAP_FIRST_WORD_MASK(start);
>                 map[index + 0] |= value << offset;
>                 map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
>                 map[index + 1] |= value >> space;
>         }
> }
>
> Or is this going to produce the same warning because we're not using an
> explicit check against the map array size?

https://godbolt.org/z/fxnsG9

It still warns about the 'map[index + 1]' access: from all I can tell,
gcc mainly complains because it cannot rule out that 'space < nbits',
and then it knows the size of 'DECLARE_BITMAP(old, 64)' and finds
that if 'index + 0' is correct, then 'index + 1' overflows that array.

      Arnd