some questions about the HVM

some questions about the HVM

[WangWei王伟]

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Hi all,
I have a question about the VMEXIT in HVM.
Does the behavior of modifying the pte of guest OS cause a vmexit.
If so ,the vmexit reason is page fault or something else ?
Look forward to you help!
Thanks!

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Re: some questions about the HVM

[Tim Deegan]

Hi, 

At 14:00 +0000 on 26 Mar (1269612028), WangWei???? wrote:
> I have a question about the VMEXIT in HVM.
> Does the behavior of modifying the pte of guest OS cause a vmexit.

When using shadow pagetables, yes (more precisely, modifying a page that
is shadowed causes a VMEXIT).  When using hardware-assisted paging (NPT
or EPT), no.

> If so ,the vmexit reason is page fault or something else ?

It's a page fault, because shadowed pages are always mapped read-only in
the shadow page tables.

Why do you want to know?

Cheers,

Tim.

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

Re: some questions about the HVM

[George Dunlap]

On Fri, Mar 26, 2010 at 5:58 PM, Tim Deegan <Tim.Deegan@citrix.com> wrote:
>> I have a question about the VMEXIT in HVM.
>> Does the behavior of modifying the pte of guest OS cause a vmexit.
>
> When using shadow pagetables, yes (more precisely, modifying a page that
> is shadowed causes a VMEXIT)

...but it's complicated.  We have an optimization called "out-of-sync"
such that modifying PTE may not cause a VMEXIT; instead, the VMEXIT
may happen when the virtual address the PTE controls is dereferenced
(akin to a TLB miss).  As Tim said, this is only in shadow mode, if
HAP (EPT or NPT) is not present / disabled.

 -George