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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

Ian.


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