From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756523Ab3KFLeb (ORCPT ); Wed, 6 Nov 2013 06:34:31 -0500 Received: from smtp.citrix.com ([66.165.176.89]:48791 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756390Ab3KFLea (ORCPT ); Wed, 6 Nov 2013 06:34:30 -0500 X-IronPort-AV: E=Sophos;i="4.93,646,1378857600"; d="scan'208";a="71102916" Message-ID: <527A2943.7050400@citrix.com> Date: Wed, 6 Nov 2013 11:34:27 +0000 From: David Vrabel User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.16) Gecko/20121215 Iceowl/1.0b1 Icedove/3.0.11 MIME-Version: 1.0 To: Anthony Liguori CC: Matt Wilson , =?UTF-8?B?Um9nZXIgUGF1IE1vbm7DqQ==?= , Ian Campbell , , , 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> <527916EE.6060104@citrix.com> <20131105200616.GA23362@u109add4315675089e695.ant.amazon.com> <878ux2vade.fsf@codemonkey.ws> In-Reply-To: <878ux2vade.fsf@codemonkey.ws> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.80.2.76] X-DLP: MIA2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/11/13 20:53, Anthony Liguori wrote: > 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. No. A m2p lookup should not be needed for DMA unmap. We only need the p2m lookup on DMA map to get the bus address -- there's should be nothing to do on the unmap. However, there is dma_mark_clean(phys_to_virt(phys), size) call in xen_unmap_single() which would require the m2p lookup but this call is not necessary as dma_mark_clean() is a no-op for anything except ia64 and we do not care about ia64. >>>>> 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. Matt, Anthony, I presume you have profiling results or performance data that support this proposed change? Can you provide them? > 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. The full changeset description for this change doesn't make sense to me. xen/blkback: Utilize the M2P override mechanism for GNTMAP_host_map Instead of doing copy grants lets do mapping grants using the M2P(and P2M) override mechanism. As all it is doing is replacing set_phys_to_machine() calls with m2p_add_override(). David