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Re: [Xen-devel] page fault handling in Xen

  • To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>, Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>
  • From: jeet <jeet_sat12@xxxxxxxxxxx>
  • Date: Mon, 5 Mar 2007 18:39:03 +0530 (IST)
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Mon, 05 Mar 2007 05:08:14 -0800
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

Hi Keir 

thanks for your reply.I have some more queries

In HVM when guest is executing then processor looks into shadow page table for 
address translation.
as shadow PT contains the guest pt page readonly mapping of guest level 1 page 
table, on which guest OS is trying to write a address of new page.    1
this would cause second VMEXit in Xen. 

so would faulting address be virtual address of that Guest PT page or of the 
shadow PT page which guest is trying to write?


> Above code will execute and in inner unlikely condition will be true as PT
> table is read only and  error is write_access
> and control will go to not_a_shadow_fault?
> so when the emulation code would be executed for the write that has been
> performed on read only guest PT for adding entry in
> guest page table by guest?
> Have I missed some thing here in understanding

Yes: guest PTs are mapped with write permissions in the guest PTs. It's only
in the shadow PTs that the guest PTs are mapped read-only. The test on
accumulated_gflags is looking at the access permissions in the guest PTs.

 -- Keir

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