From mboxrd@z Thu Jan 1 00:00:00 1970 From: Julien Grall Subject: Re: [Xen-devel] [PATCH v2] drm/xen-front: Make shmem backed display buffer coherent Date: Fri, 18 Jan 2019 11:43:32 +0000 Message-ID: <2ed681b9-c8d4-ea27-6b8b-e399ed034bda@arm.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1cfc5cb4-e481-5d71-29df-958db76fb681@epam.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org 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 List-Id: dri-devel@lists.freedesktop.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