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Re: [Xen-devel] question about atomic get_page

On 11 Jun 2005, at 05:08, Tian, Kevin wrote:

However on ia64 where _domain is 64bit, hard to use single instruction
to cover two fields. Yes, there's workaround to solve that, like adding
spinlock with some overhead. But I'd like to know the reason why _domain
and count_info should be operated together. Is there any case that
ownership changes just after checking ownership and before modifying

There'd be nothing to stop the domain pointer changing while you modify the reference count. On x86/64 we 'pickle' the domain pointer into a 32-bit field by simply masking off the high bits. Do you really need the 64-bit range? Checking the domain field after modifying the reference count would probably be okay really, it's just that a domain could get a handle on another domain's page for a short while without that other domain's permission.

 -- Keir

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