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

[Jimi Xenidis <jimix@xxxxxxxxxxxxxx>]

On Dec 18, 2006, at 4:26 AM, Keir Fraser wrote:

> On 17/12/06 5:53 pm, "Jimi Xenidis" <jimix@xxxxxxxxxxxxxx> wrote:
>
> > On PowerPC (and other big endian and/or RISC architectures) bit
> > offsets in a bitmap are actually represented by a bit-offset from an
> > element in an array rather than a bit-offset from the base memory
> > pointer,  see xen/include/asm-powerpc/bitops.h for a complete
> > explanation.
> >
> > This complicates the conversion of cpumask_t from/to xenctl_cpumap.
> >
> > The following patch allows an architecture to declare that bitops are
> > "by long" rather than "by bit" and use an alternate scheme for
> > encoding.
> If the array-element type matters on big-endian systems then the tools
> really need fixing: one libxc operation passes down a uint32_t unit;
Of the 3 usages only the tracebuffer mask uses uint32_t, maybe that  
could be repaired with the updated tracing work.
> another
> a uint64_t. Perhaps we should fix so they pass down 8-bit units (as  
> the type
> of xenctl_cpumap_t would suggest) and then we could have  
> byte_to_long_bitmap
> and long_to_byte_bitmap in Xen
Are you suggesting to still use bits in the bytes?
If so, then do you want "C" ordering in the byte or arch bit ordering?
Perhaps we simply to us the byte array as a list of CPU numbers, with  
-1 to terminate?
> (which might encourage us to pull proper
> endianness headers into Xen).
I suspect that this is more "RISC thing" than an "Endian thing" since  
it is possible for little endian arch to use the array method (like  
ARM?).
Pulling in endian headers could be avoided.

-JX






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