[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [Xen-devel] question about atomic get_page
>-----Original Message----- >From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx] >Sent: Saturday, June 11, 2005 1:36 PM >To: Tian, Kevin > > >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 Got it. And one prerequisite for this trick is that xen heap always stays in area < 4G machine memory, or else 'unpickle' will go weird then. :) Currently seems xen heap can satisfy it on all archs, including ia64. Then I will also change that field back to 32bit and everything will work same as x86. Thanks a lot, Kevin _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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