Re: [Xen-devel] reserved bit in ShadowPT

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 04:29 +0100 on 01 Sep (1251779345), Wu Bingzheng wrote:
> Hi,
> 
> Dose Shadow Page Table use reserved-bit (PFEC_reserved_bit) in Xen 3.3.0?

Yes.  It uses reserved bits in ptes to indicate entries that are known
to be either emulated-MMIO or not-present in the guest pagetables.  Then
in the fault handler it can use PFEC_reserved_bit to branch onto a fast
path for those cases without taking any locks or walking the guest
pagetables.

> In my test many page fault set reserved-bit. But I can't understand its 
> function, after reading SPT's page-fault handler.

Have a look at the definition of SH_L1E_MAGIC in arch/x86/mm/shadow/types.h

Cheers,

Tim.


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

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