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[85.223.209.22]) by smtp.gmail.com with ESMTPSA id u26-v6sm3949241lji.22.2018.12.20.03.19.04 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 20 Dec 2018 03:19:05 -0800 (PST) Subject: Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent To: Gerd Hoffmann , "Oleksandr_Andrushchenko@epam.com" Cc: =?UTF-8?Q?Noralf_Tr=c3=b8nnes?= , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, daniel.vetter@intel.com, jgross@suse.com, boris.ostrovsky@oracle.com References: <20181127103252.20994-1-andr2000@gmail.com> <17640791-5306-f7e4-8588-dd39c14e975b@tronnes.org> <20181219131452.cehks3kabcwuuk7i@sirius.home.kraxel.org> <20181219141000.k426c7o6ncsdzrn5@sirius.home.kraxel.org> From: Oleksandr Andrushchenko Message-ID: <3e24ecc5-25e1-3d5e-2092-daa95ae36cba@gmail.com> Date: Thu, 20 Dec 2018 13:19:04 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20181219141000.k426c7o6ncsdzrn5@sirius.home.kraxel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/19/18 4:10 PM, Gerd Hoffmann wrote: > Hi, > >>> Sure this actually helps? It's below 4G in guest physical address >>> space, so it can be backed by pages which are actually above 4G in host >>> physical address space ... >> Yes, you are right here. This is why I wrote about the IOMMU >> and other conditions. E.g. you can have a device which only >> expects 32-bit, but thanks to IOMMU it can access pages above >> 4GiB seamlessly. So, this is why I *hope* that this code *may* help >> such devices. Do you think I don't need that and have to remove? > I would try without that, and maybe add a runtime option (module > parameter) later if it turns out some hardware actually needs that. > Devices which can do 32bit DMA only become less and less common these > days. Good point, will remove then >>>>>> +    if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents, >>>>>> +            DMA_BIDIRECTIONAL)) { >>>>> Are you using the DMA streaming API as a way to flush the caches? >>>> Yes >>>>> Does this mean that GFP_USER isn't making the buffer coherent? >>>> No, it didn't help. I had a question [1] if there are any other better way >>>> to achieve the same, but didn't have any response yet. So, I implemented >>>> it via DMA API which helped. >>> set_pages_array_*() ? >>> >>> See arch/x86/include/asm/set_memory.h >> Well, x86... I am on arm which doesn't define that... > Oh, arm. Maybe ask on a arm list then. I know on arm you have to care > about caching a lot more, but that also is where my knowledge ends ... > > Using dma_map_sg for cache flushing looks like a sledge hammer approach > to me. It is. This is why I am so unsure this is way to go > But maybe it is needed to make xen flush the caches (xen guests > have their own dma mapping implementation, right? Or is this different > on arm than on x86?). I'll try to figure out > cheers, > Gerd > Thank you, Oleksandr