Re: [Xen-devel] [PATCH v2 24/30] tools/libxc: Modify bitmap operations to take void pointers

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Feb 10, 2016 at 10:07:12AM +0000, Andrew Cooper wrote:
> On 08/02/16 16:36, Ian Campbell wrote:
> > On Mon, 2016-02-08 at 16:23 +0000, Tim Deegan wrote:
> >> At 13:42 +0000 on 05 Feb (1454679737), Andrew Cooper wrote:
> >>> The type of the pointer to a bitmap is not interesting; it does not
> >>> affect the
> >>> representation of the block of bits being pointed to.
> >> It does affect the alignment, though.  Is this safe on ARM?
> > Good point. These constructs in the patch:
> >
> > +    const unsigned long *addr = _addr;
> >
> > Would be broken if _addr were not suitably aligned for an unsigned long.
> >
> > That probably rules out this approach unfortunately.
> 
> What about reworking libxc bitops in terms of unsigned char?  That
> should cover all alignment issues.

See 3cab67ac83b1d56c3daedd9c4adfed497a114246

"+/*
+ * xc_bitops.h has macros that do this as well - however they assume that
+ * the bitmask is word aligned but xc_cpumap_t is only guaranteed to be
+ * byte aligned and so we need byte versions for architectures which do
+ * not support misaligned accesses (which is basically everyone
+ * but x86, although even on x86 it can be inefficient).
+ */
"

> 
> ~Andrew
> 
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