Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758172AbbGQNWG (ORCPT ); Fri, 17 Jul 2015 09:22:06 -0400 Received: from smtp02.citrix.com ([66.165.176.63]:56429 "EHLO SMTP02.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758160AbbGQNWB (ORCPT ); Fri, 17 Jul 2015 09:22:01 -0400 X-IronPort-AV: E=Sophos;i="5.15,496,1432598400"; d="scan'208";a="285099287" Date: Fri, 17 Jul 2015 14:20:55 +0100 From: Stefano Stabellini X-X-Sender: sstabellini@kaball.uk.xensource.com To: Julien Grall CC: Stefano Stabellini , "Konrad Rzeszutek Wilk" , , , David Vrabel , , Boris Ostrovsky , Subject: Re: [Xen-devel] [PATCH v2 09/20] xen/biomerge: Don't allow biovec to be merge when Linux is not using 4KB page In-Reply-To: <55A7D8AD.1090102@citrix.com> Message-ID: References: <1436474552-31789-1-git-send-email-julien.grall@citrix.com> <1436474552-31789-10-git-send-email-julien.grall@citrix.com> <20150710191245.GA31063@l.oracle.com> <55A7D8AD.1090102@citrix.com> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" X-DLP: MIA1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1871 Lines: 40 On Thu, 16 Jul 2015, Julien Grall wrote: > Hi Stefano, > > On 16/07/2015 16:33, Stefano Stabellini wrote: > > On Fri, 10 Jul 2015, Konrad Rzeszutek Wilk wrote: > > > On Thu, Jul 09, 2015 at 09:42:21PM +0100, Julien Grall wrote: > > > > When Linux is using 64K page granularity, every page will be slipt in > > > > multiple non-contiguous 4K MFN (page granularity of Xen). > > > > > > But you don't care about that on the Linux layer I think? > > > > > > As in, is there an SWIOTLB that does PFN to MFN and vice-versa > > > translation? > > > > > > I thought that ARM guests are not exposed to the MFN<->PFN logic > > > and trying to figure that out to not screw up the DMA engine > > > on a PCIe device slurping up contingous MFNs which don't map > > > to contingous PFNs? > > > > Dom0 is mapped 1:1, so pfn == mfn normally, however grant maps > > unavoidably screw up the 1:1, so the swiotlb jumps in to save the day > > when a foreign granted page is involved in a dma operation. > > > > Regarding xen_biovec_phys_mergeable, we could check that all the pfn == > > mfn and return true in that case. > > I mentioned it in the commit message. Although, we would have to loop on every > pfn which is slow on 64KB (16 times for every page). Given the biovec is > called often, I don't think we can do a such things. We would have to run some benchmarks, but I think it would still be a win. We should write an ad-hoc __pfn_to_mfn translation function that operates on a range of pfns and simply checks whether an entry is present in that range. It should be just as fast as __pfn_to_mfn. I would definitely recommend it. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/