From mboxrd@z Thu Jan  1 00:00:00 1970
From: George Dunlap <dunlapg@gmail.com>
Subject: Re: some questions about the HVM
Date: Tue, 30 Mar 2010 10:57:37 +0100
Message-ID: <de76405a1003300257g3f5d4618k7f26bc145d74115@mail.gmail.com>
References: <e85906171003260700o7b607f64hca5d97e65513756f@mail.gmail.com>
	<20100326165856.GA4975@whitby.uk.xensource.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Return-path: <xen-devel-bounces@lists.xensource.com>
In-Reply-To: <20100326165856.GA4975@whitby.uk.xensource.com>
List-Unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
List-Post: <mailto:xen-devel@lists.xensource.com>
List-Help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-Subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
Sender: xen-devel-bounces@lists.xensource.com
Errors-To: xen-devel-bounces@lists.xensource.com
To: Tim Deegan <Tim.Deegan@citrix.com>
Cc: "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, WangWei???? <wangwei.cs@gmail.com>
List-Id: xen-devel@lists.xenproject.org

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