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Re: [Xen-devel] About shadow flag(PG_refcounts)

from [Tim Deegan]

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To:	Li Yaqiong <liyaq04@xxxxxxxxxx>
Subject:	Re: [Xen-devel] About shadow flag(PG_refcounts)
From:	Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date:	Mon, 12 Jan 2009 09:51:54 +0000
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Hi,

At 11:15 +0800 on 12 Jan (1231758914), Li Yaqiong wrote:
> Hi
>          In Xen source code, a flag called PG_refcounts is used. Its
> explanation is "Refcounts based on shadow tables instead of guest
> tables". I can't understand it. Who can explain this flags in detail?

If this flag is zero, the shadow pagetables do not do reference counting
of the pages that they map (because the normal PV pagetable operations
will have done it already when the guest registered its pagetables).

If the flag is one, the shadow pagetables _do_ reference-count every
mapping they make of a guest page (for example in HVM guests, where
there's no other reference counting of the guest's use of memory).

Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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