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

To: "Hollis Blanchard" <hollisb@xxxxxxxxxx>, "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] RFC: 32 bits as smallest atomic size.
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Tue, 29 Mar 2005 23:00:57 +0100
Cc: Jimi Xenidis <jimix@xxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Thread-index: AcU0qCNIrJOmzQhhR8qAiy01laby0wAAE+6g
Thread-topic: [Xen-devel] RFC: 32 bits as smallest atomic size.
> 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. 

This seems a better fix, at least to me.

Ian

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