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Re: [RFC] Re: [Xen-devel] [PATCH] fix xenctl_cpumap translation to handle bitops accessed like arrays

On Jan 17, 2007, at 7:20 AM, Keir Fraser wrote:

On 16/1/07 23:26, "Jimi Xenidis" <jimix@xxxxxxxxxxxxxx> wrote:

Sorry for taking so long with this, please comment on the following:

The following patch adds the ability to convert form "long" based
bitmaps to "byte" base bitmaps for portability. This is only has the
hypervisor side, but the definitions are such that the same prototype
can be use by XC and Xen.

Sadly it adds a copy on the xenctl_cpumap_to_cpumask(), but it minimally
impacts the common paths.

Fine interface. On the Xen side the prototypes should go in xen/ bitmap.h and
the implementations in common/bitmap.c.


I've added the Linux byteorder
functions so in bitmap.c you ifdef on __BIG_ENDIAN and then use le32_to_cpu
or le64_to_cpu (depending on BITS_PER_LONG).

There's a question over whether
we should do this transformation in-place on the bitmap, or use the copying
semantics that you've chosen. I guess the latter is cleaner.

A copy will almost always be necessary since the source or destination (always the long) may actually be active and could have come from anywhere, where the byte is most likely local.

Actually, your
'no-op' long_to_byte_bitmap() implementation does not memcpy -- why is that?

This is a little (possibly dirty) attempt to avoid a copy if no conversion is required.
As long as the caller uses the return value all is well.
I'd happy to make it a memcpy() and feel better about it.

On the guest side you'll need to dup the function prototypes and
implementations in libxenctrl. Trying to share prototypes or anything via the public headers is just skanky. Add a comment to xenctl_cpumap to explain the format (explain it is 'little endian' or 'byte oriented' with an ascii
diagram) and leave it at that.


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