RE: [Xen-devel] RFC: 32 bits as smallest atomic size.

[Hollis Blanchard <hollisb@xxxxxxxxxx>]

On Tue, 2005-03-29 at 23:00 +0100, Ian Pratt wrote:
> > Right now there is a single problematic cmpxchg user, 
> > grant_entry_t, and the impact of the fix is tiny (make the 
> > 'flags' member 4 bytes). Is that too large an inconvenience?
> 
> That would make grant table entries 12 bytes rather than 8 bytes, which
> is not ideal given that you can end up needing quite a lot of them,
> particularly when the cacheline packing is considered. 
> 
> Perhaps the best thing to do would be to change that particular instance
> of cmpxchg into cmpxchg_16, where we handle the subword case on
> architectures which can't do it directly by rounding down the address,
> doing a read, insert, and cmpxchg. 

If you look closely, that code is a hack that Keir mentioned would be
replaced soon. Two 16-bit quantities are being concatenated (in an
endian-unsafe manner), then passed as a 32-bit quantity to cmpxchg,
using the address of the first 16-bit value.

I was looking for a more general policy though. There are a number of
atomic operations (including xchg, cmpxchg, bitops' *_bit, and atomic_*)
that will require extra code (to write and run) on many architectures,
if the types used are not native word size. Why shouldn't Xen follow
Linux's example here?

-- 
Hollis Blanchard
IBM Linux Technology Center


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