Re: [Xen-devel] question about atomic get_page

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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
> count_info?
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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