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Tue, 29 Jan 2019 23:56:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=codeaurora.org; s=default; t=1548806171; bh=3E39rNK8WQfnmWhN+okT8Vvmmb/HXZD26rZ/X0hdh0Q=; h=Date:From:To:cc:Subject:In-Reply-To:References:From; b=eqouPe/3rjVU1uFf653gYATT6UXa43w/iOopbJUYEmY4FIPK09z2VzvXbZRE+IXoZ kSn8Y1NDK5DE4vM/HXWiIz3j06nQxTNkYWIhXuIKhZ2nWBeuKmwVvyrrDowFxfwEBI 9o93jfpyLKeN/vA5c652D/gi2wJmRe1pbZwGdfQc= DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org A6D87604BE Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=lmark@codeaurora.org Date: Tue, 29 Jan 2019 15:56:10 -0800 (PST) From: Liam Mark X-X-Sender: lmark@lmark-linux.qualcomm.com To: "Skidanov, Alexey" cc: Laura Abbott , Greg KH , "devel@driverdev.osuosl.org" , "tkjos@android.com" , "rve@android.com" , "linux-kernel@vger.kernel.org" , "maco@android.com" , "sumit.semwal@linaro.org" Subject: RE: [PATCH v3] staging: android: ion: Add implementation of dma_buf_vmap and dma_buf_vunmap In-Reply-To: <040863540BC4D141BEB177532350882876A5692E@hasmsx108.ger.corp.intel.com> Message-ID: References: <1517400222-2854-1-git-send-email-alexey.skidanov@intel.com> <20180131130047.GA22917@kroah.com> <89be45dd-ac0e-4efb-2b21-e6c07b0cd66e@intel.com> <7d836085-cd8d-300b-56be-e8db4ff37afc@intel.com> <67f288db-d9ea-2d45-ddcf-a15dbff316f5@intel.com> <3740948f-02be-cf7a-bc41-54b4fd195103@intel.com> <040863540BC4D141BEB177532350882876A5692E@hasmsx108.ger.corp.intel.com> User-Agent: Alpine 2.10 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 4 Jan 2019, Skidanov, Alexey wrote: > > > > -----Original Message----- > > From: Liam Mark [mailto:lmark@codeaurora.org] > > Sent: Friday, January 04, 2019 19:42 > > To: Skidanov, Alexey > > Cc: Laura Abbott ; Greg KH ; > > devel@driverdev.osuosl.org; tkjos@android.com; rve@android.com; linux- > > kernel@vger.kernel.org; maco@android.com; sumit.semwal@linaro.org > > Subject: Re: [PATCH v3] staging: android: ion: Add implementation of dma_buf_vmap and > > dma_buf_vunmap > > > > On Tue, 18 Dec 2018, Alexey Skidanov wrote: > > > > > >>> I was wondering if we could re-open the discussion on adding support to > > > >>> ION for dma_buf_vmap. > > > >>> It seems like the patch was not taken as the reviewers wanted more > > > >>> evidence of an upstream use case. > > > >>> > > > >>> Here would be my upstream usage argument for including dma_buf_vmap > > > >>> support in ION. > > > >>> > > > >>> Currently all calls to ion_dma_buf_begin_cpu_access result in the creation > > > >>> of a kernel mapping for the buffer, unfortunately the resulting call to > > > >>> alloc_vmap_area can be quite expensive and this has caused a performance > > > >>> regression for certain clients when they have moved to the new version of > > > >>> ION. > > > >>> > > > >>> The kernel mapping is not actually needed in ion_dma_buf_begin_cpu_access, > > > >>> and generally isn't needed by clients. So if we remove the creation of the > > > >>> kernel mapping in ion_dma_buf_begin_cpu_access and only create it when > > > >>> needed we can speed up the calls to ion_dma_buf_begin_cpu_access. > > > >>> > > > >>> An additional benefit of removing the creation of kernel mappings from > > > >>> ion_dma_buf_begin_cpu_access is that it makes the ION code more secure. > > > >>> Currently a malicious client could call the DMA_BUF_IOCTL_SYNC IOCTL with > > > >>> flags DMA_BUF_SYNC_END multiple times to cause the ION buffer kmap_cnt to > > > >>> go negative which could lead to undesired behavior. > > > >>> > > > >>> One disadvantage of the above change is that a kernel mapping is not > > > >>> already created when a client calls dma_buf_kmap. So the following > > > >>> dma_buf_kmap contract can't be satisfied. > > > >>> > > > >>> /** > > > >>> * dma_buf_kmap - Map a page of the buffer object into kernel address > > > >>> space. The > > > >>> * same restrictions as for kmap and friends apply. > > > >>> * @dmabuf: [in] buffer to map page from. > > > >>> * @page_num: [in] page in PAGE_SIZE units to map. > > > >>> * > > > >>> * This call must always succeed, any necessary preparations that might > > > >>> fail > > > >>> * need to be done in begin_cpu_access. > > > >>> */ > > > >>> > > > >>> But hopefully we can work around this by moving clients to dma_buf_vmap. > > > >> I think the problem is with the contract. We can't ensure that the call > > > >> is always succeeds regardless the implementation - any mapping might > > > >> fail. Probably this is why *all* clients of dma_buf_kmap() check the > > > >> return value (so it's safe to return NULL in case of failure). > > > >> > > > > > > > > I think currently the call to dma_buf_kmap will always succeed since the > > > > DMA-Buf contract requires that the client first successfully call > > > > dma_buf_begin_cpu_access(), and if dma_buf_begin_cpu_access() succeeds > > > > then dma_buf_kmap will succeed. > > > > > > > >> I would suggest to fix the contract and to keep the dma_buf_kmap() > > > >> support in ION. > > > > > > > > I will leave it to the DMA-Buf maintainers as to whether they want to > > > > change their contract. > > > > > > > > Liam > > > > > > > > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, > > > > a Linux Foundation Collaborative Project > > > > > > > > > > Ok. We need the list of the clients using the ION in the mainline tree. > > > > > > > Looks to me like the only functions which might be calling > > dma_buf_kmap/dma_buf_kunmap on ION buffers are > > tegra_bo_kmap/tegra_bo_kunmap, I assume Tegra is used in some Android > > automotive products. > > > > Looks like these functions could be moved over to using > > dma_buf_vmap/dma_buf_vunmap but it wouldn't be very clean and would add a > > performance hit. > > > > Liam > > > > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, > > a Linux Foundation Collaborative Project > > I'm a little bit confused. Why making the buffer accessible by CPU (mapping the buffer) > and making the content of the buffer valid (coherent) are so tightly coupled in DMA-BUF? > Hi Sumit, Hope you are feeling better. I was wondering if you would be open to changes to to the DMA-BUF contract so that we can remove the creation of kernel mappings in begin_cpu_access. This would have the benefit of improving the performance of begin_cpu_access and removing the ability for userspace to add and remove kernel mappings. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project