From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: Regarding page table management changes from Xen v1to Xen v2 (and v3) Date: Wed, 26 Apr 2006 13:19:57 -0500 Message-ID: <444FB9CD.4020000@us.ibm.com> References: <444FB8BD.4070708@us.ibm.com> <56e332805d874d1291f6dda3a1bed157@cl.cam.ac.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <56e332805d874d1291f6dda3a1bed157@cl.cam.ac.uk> 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: Ian Pratt , xen-devel@lists.xensource.com, Himanshu Raj List-Id: xen-devel@lists.xenproject.org Keir Fraser wrote: > > On 26 Apr 2006, at 19:15, Anthony Liguori wrote: > >>> No, these singleton updates on the Linux demand fault path are issued >>> with the update_va_mapping hypercall. This is possible because the >>> virtual address that the pte maps is known, so its possible for Xen to >>> use a linear pagetable to do the update rather than creating an >>> on-demand mapping to the domain's pagetable page. This is an important >>> optimization on 32b where Xen address space is tight. >>> >> >> Just to make sure I understand, we do the linear mapping for >> performance (so we don't have to keep remapping) but not necessarily >> to save VA space since the linear mapping uses a fixed amount of space. >> >> Is there any reason that we would strictly require a linear mapping >> (other than it being convenient)? > > On 64b we could use the 1:1 mapping of all physmem that Xen has. > Obviously that is infeasible on 32b. In fact the interface (passing a > virtual address) pretty much means we have to use a linear mapping in > both cases. Couldn't we temporarily map the page table page? Not that we would actually want to do that. Just trying to confirm my understand. Regards, Anthony Liguori > -- Keir >