From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755767AbYIIPsS (ORCPT ); Tue, 9 Sep 2008 11:48:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753393AbYIIPsI (ORCPT ); Tue, 9 Sep 2008 11:48:08 -0400 Received: from gw.goop.org ([64.81.55.164]:36626 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753370AbYIIPsI (ORCPT ); Tue, 9 Sep 2008 11:48:08 -0400 Message-ID: <48C69AB6.20500@goop.org> Date: Tue, 09 Sep 2008 08:48:06 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.16 (X11/20080723) MIME-Version: 1.0 To: Keir Fraser CC: Avi Kivity , Ingo Molnar , Xen Devel , Andi Kleen , linux-kernel@vger.kernel.org, "H. Peter Anvin" Subject: Re: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO mappings References: In-Reply-To: X-Enigmail-Version: 0.95.7 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Keir Fraser wrote: > That's basically what our _PAGE_IO flag (in our old Linux patchset) means. > We use it to cause pte_pfn() to return an invalid pfn and hence avoid > reference counting that way. Since kernel mappings are never reference > counted (I think?) perhaps we could use _PAGE_SPECIAL even if it is > restricted to use on user mappings. Well, _PAGE_IOMAP's most important semantic from Xen's perspective is that the frame number is considered to already be an MFN and so isn't converted. It may be that _PAGE_SPECIAL is also useful for its "no refcount" properties, but we could set both in that case. At present we can't set _PAGE_SPECIAL on kernel mappings because we assume its usermode only, and its shared with _PAGE_CPA_TEST which is kernel-only. Most _PAGE_IOMAP mappings are in kernel space, but I wouldn't discount user mappings if we end up allowing direct access to passthrough devices. J