From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755128Ab3KEQEW (ORCPT ); Tue, 5 Nov 2013 11:04:22 -0500 Received: from smtp.citrix.com ([66.165.176.89]:58347 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751532Ab3KEQEU (ORCPT ); Tue, 5 Nov 2013 11:04:20 -0500 X-IronPort-AV: E=Sophos;i="4.93,640,1378857600"; d="scan'208";a="70768381" Message-ID: <527916EE.6060104@citrix.com> Date: Tue, 5 Nov 2013 17:03:58 +0100 From: =?UTF-8?B?Um9nZXIgUGF1IE1vbm7DqQ==?= User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 MIME-Version: 1.0 To: Ian Campbell CC: Konrad Rzeszutek Wilk , , David Vrabel , Matt Wilson , Subject: Re: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops is not set 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> <1383664090.13961.88.camel@kazak.uk.xensource.com> In-Reply-To: <1383664090.13961.88.camel@kazak.uk.xensource.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-DLP: MIA1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/11/13 16:08, Ian Campbell wrote: > 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. Yes, you are completely right, then I have to figure out why blkback works fine with this patch applied (or at least it seems to work fine).