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 E07C3C43387 for ; Fri, 18 Jan 2019 11:43:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B02B820823 for ; Fri, 18 Jan 2019 11:43:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727559AbfARLnh (ORCPT ); Fri, 18 Jan 2019 06:43:37 -0500 Received: from foss.arm.com ([217.140.101.70]:55476 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727517AbfARLnh (ORCPT ); Fri, 18 Jan 2019 06:43:37 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5880A80D; Fri, 18 Jan 2019 03:43:36 -0800 (PST) Received: from [10.1.196.50] (e108454-lin.cambridge.arm.com [10.1.196.50]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 65C8C3F557; Fri, 18 Jan 2019 03:43:34 -0800 (PST) Subject: Re: [Xen-devel] [PATCH v2] drm/xen-front: Make shmem backed display buffer coherent To: Oleksandr Andrushchenko , Christoph Hellwig Cc: "jgross@suse.com" , Oleksandr Andrushchenko , "linux-kernel@vger.kernel.org" , "dri-devel@lists.freedesktop.org" , "noralf@tronnes.org" , Gerd Hoffmann , "daniel.vetter@intel.com" , "xen-devel@lists.xenproject.org" , "boris.ostrovsky@oracle.com" , Stefano Stabellini References: <20190115140420.4652-1-andr2000@gmail.com> <20190116063002.jq3aas6ofhaogmyk@sirius.home.kraxel.org> <20190116063631.GA10881@infradead.org> <042fe794-dec6-6d9d-5b68-6f142790f5c7@epam.com> <20190117091815.GA7090@infradead.org> <1cfc5cb4-e481-5d71-29df-958db76fb681@epam.com> From: Julien Grall Message-ID: <2ed681b9-c8d4-ea27-6b8b-e399ed034bda@arm.com> Date: Fri, 18 Jan 2019 11:43:32 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.0 MIME-Version: 1.0 In-Reply-To: <1cfc5cb4-e481-5d71-29df-958db76fb681@epam.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (+ Stefano) Hi, Sorry for jumping late in the conversation. On 18/01/2019 09:40, Oleksandr Andrushchenko wrote: > On 1/17/19 11:18 AM, Christoph Hellwig wrote: >> On Wed, Jan 16, 2019 at 06:43:29AM +0000, Oleksandr Andrushchenko wrote: >>>> This whole issue keeps getting more and more confusing. >>> Well, I don't really do DMA here, but instead the buffers in >>> question are shared with other Xen domain, so effectively it >>> could be thought of some sort of DMA here, where the "device" is >>> that remote domain. If the buffers are not flushed then the >>> remote part sees some inconsistency which in my case results >>> in artifacts on screen while displaying the buffers. >>> When buffers are allocated via DMA API then there are no artifacts; >>> if buffers are allocated with shmem + DMA mapping then there are no >>> artifacts as well. >>> The only offending use-case is when I use shmem backed buffers, >>> but do not flush them >> The right answer would be to implement cache maintainance hooks for >> this case in the Xen arch code. These would basically look the same >> as the low-level cache maintainance used by the DMA ops, but without >> going through the DMA mapping layer, in fact they should probably >> reuse the same low-level assembly routines. >> >> I don't think this is the first usage of such Xen buffer sharing, so >> what do the other users do? > I'll have to get even deeper into it. Initially I > looked at the code, but didn't find anything useful. > Or maybe I have just overlooked obvious things there From Xen on Arm ABI: "All memory which is shared with other entities in the system (including the hypervisor and other guests) must reside in memory which is mapped as Normal Inner Write-Back Outer Write-Back Inner-Shareable. This applies to: - hypercall arguments passed via a pointer to guest memory. - memory shared via the grant table mechanism (including PV I/O rings etc). - memory shared with the hypervisor (struct shared_info, struct vcpu_info, the grant table, etc). " So you should not need any cache maintenance here. Can you provide more details on the memory attribute you use for memory shared in both the backend and frontend? Cheers, > > Thank you, > Oleksandr > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xenproject.org > https://lists.xenproject.org/mailman/listinfo/xen-devel > -- Julien Grall