From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Egger, Christoph" Subject: Re: [PATCH 00/11] Alternate p2m: support multiple copies of host p2m Date: Thu, 5 Mar 2015 14:45:49 +0100 Message-ID: <54F85E0D.6070108@amazon.de> References: <1420838801-11704-1-git-send-email-edmund.h.white@intel.com> <54B56B79.3010109@citrix.com> <54B579CD.60804@intel.com> <54B583FC.4060800@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <54B583FC.4060800@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andrew Cooper , Ed White , xen-devel@lists.xen.org Cc: keir@xen.org, ian.campbell@citrix.com, tim@xen.org, ian.jackson@eu.citrix.com, jbeulich@suse.com, Tamas K Lengyel List-Id: xen-devel@lists.xenproject.org On 2015/01/13 21:45, Andrew Cooper wrote: > On 13/01/15 20:02, Ed White wrote: >> On 01/13/2015 11:01 AM, Andrew Cooper wrote: >> >>> I can't think of any reasonable case where the alternate p2m would want >>> mappings different to the host p2m. That is to say, an altp2m will map >>> the same set of mfns to make a guest physical address space, but may >>> differ in page permissions and possibly p2m types. >>> >> The set of mfn's is the same, but I do allow gfn->mfn mappings to be >> modified under certain circumstances. One use of this is to point the >> same VA to different physical pages (with different access permissions) >> in different p2m's to hide memory changes. > > What is the practical use of being able to play paging tricks like this > behind a VMs back? > I can imagine to reduce overhead for PV frontend/backend driver communication by saying the "mfn" is dom0's "gmfn". Christoph