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[Xen-devel] Re: one question to p2m table entry type

To: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>
Subject: [Xen-devel] Re: one question to p2m table entry type
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Tue, 18 May 2010 12:04:55 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>
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At 09:17 +0100 on 05 May (1273051073), Jiang, Yunhong wrote:
> Tim/Keir, I noticed that when translatiing p2m table type and p2m pte entry 
> flags, there are difference handling for x86_64 and x32 like:
> in p2m_type_to_flags:
> #ifdef __x86_64__
>     flags = (unsigned long)(t & 0x3fff) << 9;
> #else
>     flags = (t & 0x7UL) << 9;
> #endif
> in p2m_flags_to_type:
>     /* Type is stored in the "available" bits */
> #ifdef __x86_64__
>     return (flags >> 9) & 0x3fff;
> #else
>     return (flags >> 9) & 0x7;
> But since we don't support pure 32 bit xen hypervisor any more, and
> for 32 PAE, we are sure have enough bit to keep these flags, why do we
> need these special handling? Are there any special reason for it?

The Intel SDMs (section 3.8.5, figure 3-20 in the copy in front of me)
only define three available bits in PAE PTEs; all bits above MAXPHYADDR
are reserved.  If we can rely on the manuals being wrong about that, we
can extend the number of p2m types on 32-bit XEN. :)



Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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