Re: [Xen-devel] [PATCH] xen: fix alignment for bitops

[Vladimir Murzin <murzin.v@xxxxxxxxx>]

On Tue, Apr 15, 2014 at 10:15 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Tue, 2014-04-15 at 09:49 +0100, Jan Beulich wrote:
>> >>> On 15.04.14 at 10:29, <murzin.v@xxxxxxxxx> wrote:
>> > On Tue, Apr 15, 2014 at 8:35 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> >> only place you'd need to alter - we simply assume bitops on 32-bit
>> >> aligned quantities to work.
>> >>
>> >
>> > You do, but reality seems to be different, apart from arm64 looks like
>> > ppc64 has the same alignment requirement, I'm not aware about other
>> > 64-bit implementations... but, is it really possible to convince all
>> > these people to change the implementation? I guess the answer would be
>> > "use unsigned long", or like that :)
>>
>> That's no the point. The point is that if the arch has such
>> requirements, the arch-specific bitmap manipulation functions
>> should be written such that generic code works with the present
>> assumptions.
>
> In this case the interface which arch-specific code has contracted to
> expose to the generic code is:
>         extern unsigned long find_first_bit(const unsigned long
>         *addr, ...
> i.e. the bitmask is unsigned long (and this is the case for the majority
> of archs in Linux AFAICS, include asm-generic, note that this differs to
> Xen which IIRC uses void * here)
>
> So it is reasonable IMHO for the Linux arch code to except to be passed
> something which obeys the alignment rules for an unsigned long.
>
> I'm not convinced by the use of __aligned here -- I think it would be
> better to just use unsigned long.

It was original idea, but I was in doubt that it didn't break
interface... Should I send v2 or wait for other proposals?

Another option would be implement u32 bitops for Xen, may be based on
generic Linux implementation... so there would not be any dependencies
on arches.. don't have idea how bitops performance is critical for
Xen...

Vladimir

>
> Ian.
>

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