From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755319Ab3KEUx3 (ORCPT ); Tue, 5 Nov 2013 15:53:29 -0500 Received: from mail-pb0-f46.google.com ([209.85.160.46]:33845 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754865Ab3KEUx0 convert rfc822-to-8bit (ORCPT ); Tue, 5 Nov 2013 15:53:26 -0500 From: Anthony Liguori To: Matt Wilson , Roger Pau =?utf-8?Q?Monn=C3=A9?= Cc: Ian Campbell , Konrad Rzeszutek Wilk , linux-kernel@vger.kernel.org, David Vrabel , Matt Wilson , xen-devel@lists.xen.org Subject: Re: [Xen-devel] [PATCH] grant-table: don't set m2p override if kmap_ops is not set In-Reply-To: <20131105200616.GA23362@u109add4315675089e695.ant.amazon.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> <1383664090.13961.88.camel@kazak.uk.xensource.com> <527916EE.6060104@citrix.com> <20131105200616.GA23362@u109add4315675089e695.ant.amazon.com> User-Agent: Notmuch/0.15.2+202~g0c4b8aa (http://notmuchmail.org) Emacs/23.3.1 (x86_64-pc-linux-gnu) Date: Tue, 05 Nov 2013 12:53:17 -0800 Message-ID: <878ux2vade.fsf@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Matt Wilson writes: > On Tue, Nov 05, 2013 at 05:03:58PM +0100, Roger Pau Monné wrote: >> 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). > > blkback also works for me when testing a similar patch. I'm still > confused. One thing with your proposed patch: I'm not sure that you're > putting back the correct mfn. It's perfectly fine to store a foreign pfn in the m2p table. The m2p override table is used by the grant device to allow a reverse lookup of the real mfn to a pfn even if it's foreign. blkback doesn't actually need this though. This was introduced in: commit 5dc03639cc903f887931831d69895facb5260f4b Author: Konrad Rzeszutek Wilk Date: Tue Mar 1 16:46:45 2011 -0500 xen/blkback: Utilize the M2P override mechanism for GNTMAP_host_map Purely as an optimization. In practice though due to lock contention it slows things down. I think an alternative would be to use a read/write lock instead of just a spinlock since it's the read path that is the most hot. I haven't tested that yet though. Regards, Anthony Liguori > > Adding Anthony to the thread. > > --msw