From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keir Fraser Subject: Re: Live migration leaves page tables read-only? Date: Sat, 09 Dec 2006 09:34:56 +0000 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" 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 , Ian Pratt , John Byrne Cc: xen-devel , Christian Limpach , Joe Bonasera List-Id: xen-devel@lists.xenproject.org On 9/12/06 9:22 am, "Keir Fraser" wrote: >> Although your patch works today, it will break when we add PSE (super >> page) support for PV guests as it will confuse PROT_NONE with PSE. >> Assuming PROT_NONE only makes sense for L1 entries, we can probably gate >> the tests on whether the page table page is an L1 or not to fix this. >> >> However, it does point out an issue for other OSes: Taking this patch >> effectively makes Linux's PROT_NONE (flags 0x80 for a not present PTE) >> part of the Xen API. We need to find out whether this is compatible with >> *BSD and Solaris' use of flags for not present ptes. > > If _PAGE_PRESENT is clear then the other N-1 bits can be assumed available > for things like swapcache info. Making assumptions about not-present PTEs is > not really tenable. Speaking more constructively we could have a pte_active_mask communicated via elfnote or xenbus (or some other way) which the tools would apply to PTEs to determine if they contain an MFN. Default would be 0x1. -- Keir