From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C643DC10F03 for ; Thu, 25 Apr 2019 09:01:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 91EF5214C6 for ; Thu, 25 Apr 2019 09:01:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727765AbfDYJBr convert rfc822-to-8bit (ORCPT ); Thu, 25 Apr 2019 05:01:47 -0400 Received: from prv1-mh.provo.novell.com ([137.65.248.33]:52075 "EHLO prv1-mh.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726039AbfDYJBr (ORCPT ); Thu, 25 Apr 2019 05:01:47 -0400 Received: from INET-PRV1-MTA by prv1-mh.provo.novell.com with Novell_GroupWise; Thu, 25 Apr 2019 03:01:46 -0600 Message-Id: <5CC177740200007800228EB2@prv1-mh.provo.novell.com> X-Mailer: Novell GroupWise Internet Agent 18.1.0 Date: Thu, 25 Apr 2019 03:01:40 -0600 From: "Jan Beulich" To: "Juergen Gross" Cc: "Stefano Stabellini" , , "xen-devel" , "Boris Ostrovsky" , "Konrad Rzeszutek Wilk" , Subject: Re: [Xen-devel] [PATCH 3/3] xen/swiotlb: remember having called xen_create_contiguous_region() References: <20190423105457.17502-1-jgross@suse.com> <20190423105457.17502-4-jgross@suse.com> In-Reply-To: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> On 23.04.19 at 20:36, wrote: > On 23/04/2019 19:05, Stefano Stabellini wrote: >> On Tue, 23 Apr 2019, Juergen Gross wrote: >>> Instead of always calling xen_destroy_contiguous_region() in case the >>> memory is DMA-able for the used device, do so only in case it has been >>> made DMA-able via xen_create_contiguous_region() before. >>> >>> This will avoid a lot of xen_destroy_contiguous_region() calls for >>> 64-bit capable devices. >>> >>> As the memory in question is owned by swiotlb-xen the PG_owner_priv_1 >>> flag of the first allocated page can be used for remembering. >> >> Although the patch looks OK, this sentence puzzles me. Why do you say >> that the memory in question is owned by swiotlb-xen? Because it was >> returned by xen_alloc_coherent_pages? Both the x86 and the Arm >> implementation return fresh new memory, hence, it should be safe to set >> the PageOwnerPriv1 flag? >> >> My concern with this approach is with the semantics of PG_owner_priv_1. >> Is a page marked with PG_owner_priv_1 only supposed to be used by the >> owner? > > The owner of the page is free to use the flag. > > Like Grant pages are marked by the grant driver using this flag. And > Xen page tables are using it in PV-guests for indicating a "Pinned" > page table. Considering the background of the series, isn't such multi-purpose use of the flag a possible problem? You're already suspecting a wrong call into here. The function finding the flag set (but for another reason) might add to the confusion. But I realize there are only so many page flags available. Perhaps the freeing function should, first thing, check the handed space actually matches the criteria (within dma_mask and contiguous)? Jan