From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755143Ab3KEPIa (ORCPT ); Tue, 5 Nov 2013 10:08:30 -0500 Received: from smtp02.citrix.com ([66.165.176.63]:31187 "EHLO SMTP02.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754551Ab3KEPI3 (ORCPT ); Tue, 5 Nov 2013 10:08:29 -0500 X-IronPort-AV: E=Sophos;i="4.93,640,1378857600"; d="scan'208";a="68176507" Message-ID: <1383664090.13961.88.camel@kazak.uk.xensource.com> Subject: Re: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops is not set From: Ian Campbell To: Roger Pau =?ISO-8859-1?Q?Monn=E9?= CC: Konrad Rzeszutek Wilk , , David Vrabel , Matt Wilson , Date: Tue, 5 Nov 2013 15:08:10 +0000 In-Reply-To: <5279085E.7070703@citrix.com> References: <1383650649-13971-1-git-send-email-roger.pau@citrix.com> <5278E64C.40200@citrix.com> <527904EC.9090402@citrix.com> <20131105145607.GB25836@phenom.dumpdata.com> <5279085E.7070703@citrix.com> Organization: Citrix Systems, Inc. Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.4-3 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Originating-IP: [10.30.203.1] X-DLP: MIA2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2013-11-05 at 16:01 +0100, Roger Pau Monné wrote: > On 05/11/13 15:56, Konrad Rzeszutek Wilk wrote: > > On Tue, Nov 05, 2013 at 03:47:08PM +0100, Roger Pau Monné wrote: > >> On 05/11/13 13:36, David Vrabel wrote: > >>> On 05/11/13 11:24, Roger Pau Monne wrote: > >>>> IMHO there's no reason to set a m2p override if the mapping is done in > >>>> kernel space, so only set the m2p override when kmap_ops is set. > >>> > >>> Can you provide a more detailed reasoning about why this is safe? > >> > >> To tell the truth, I don't understand why we need to use the m2p > >> override for kernel space only mappings, my understanding is that this > >> m2p override is needed for user space mappings only (where we actually > >> end up doing two mappings, one in kernel space and one in user space). > >> For kernel space I don't see why we need to do anything else than > >> setting the right p2m translation. > > > > We needed the m2p when doing DMA operations. As the driver would > > want the bus address (so p2m) and then when unmapping the DMA we > > only get the bus address - so we needed to do a m2p lookup. > > OK, we need a m2p (that we already have in machine_to_phys_mapping), > what I don't understand is why we need the m2p override. The m2p is a host global table. For a foreign page grant mapped into the current domain the m2p will give you the foreign (owner) domain's p from the m, not the local one. Ian.