From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gianluca Guida Subject: Re: Weekly VMX status report. Xen: #18846 & Xen0: #749 Date: Fri, 12 Dec 2008 23:30:37 +0000 Message-ID: <4942F41D.6060702@eu.citrix.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Keir Fraser Cc: "Li, Haicheng" , "'xen-devel@lists.xensource.com'" , "Li, Xin" List-Id: xen-devel@lists.xenproject.org Keir Fraser wrote: > Is there any guest that actually cares about having EFER_NX really cleared? > Presumably the only way of detecting this would be reserved-bit page faults, > which no OS is likely to want to deliberately cause? Yes, no OS we've actually experienced at the moment rely on reserved bit faults (with the most notable exception of Tim's fast path for MMIO and non present pages in Xen's shadow entries). I am sure about this for a very simple reason: -- some kind of secret I would like to share with you and xen-devel -- shadow code doesn't check at all for reserved bits when propagating changes from guest to shadows, so we never propagate reserved bit faults to guests. [working on this] > There's been some talk of NX'ing up Xen's data areas. In that case we > *would* need NX enabled always in host mode. Would it actually be worth > enabling/disabling on vmexit/vmentry? > > SVM actually does automatically save/restore EFER on vmentry/vmexit. Could > we use VMX's MSR load/save support for the same effect? Would it be slow, or > interact badly with the existing support for switching EFER.LME? AFAIK, this should be slow. Thanks, Gianluca