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RE: [Xen-devel] question about atomic get_page

To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] question about atomic get_page
From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date: Sat, 11 Jun 2005 13:55:41 +0800
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Thread-topic: [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
>> to cover two fields. Yes, there's workaround to solve that, like
>> 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,

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