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Thu, 7 Jan 2021 18:00:39 GMT Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserp3020.oracle.com with ESMTP id 35v1fbhu8s-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 07 Jan 2021 18:00:39 +0000 Received: from abhmp0013.oracle.com (abhmp0013.oracle.com [141.146.116.19]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id 107I0YBv019522; Thu, 7 Jan 2021 18:00:34 GMT Received: from char.us.oracle.com (/10.152.32.25) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Thu, 07 Jan 2021 18:00:33 +0000 Received: by char.us.oracle.com (Postfix, from userid 1000) id 56A6B6A00C3; Thu, 7 Jan 2021 13:00:32 -0500 (EST) Date: Thu, 7 Jan 2021 13:00:32 -0500 From: Konrad Rzeszutek Wilk To: Claire Chang Subject: Re: [RFC PATCH v3 5/6] dt-bindings: of: Add restricted DMA pool Message-ID: <20210107180032.GB16519@char.us.oracle.com> References: <20210106034124.30560-1-tientzu@chromium.org> <20210106034124.30560-6-tientzu@chromium.org> <20210106185757.GB109735@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.1 (2017-09-22) X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9857 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=999 phishscore=0 suspectscore=0 spamscore=0 bulkscore=0 adultscore=0 mlxscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2101070105 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9857 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 suspectscore=0 mlxscore=0 bulkscore=0 priorityscore=1501 impostorscore=0 clxscore=1015 lowpriorityscore=0 mlxlogscore=999 malwarescore=0 spamscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2101070105 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: heikki.krogerus@linux.intel.com, peterz@infradead.org, grant.likely@arm.com, paulus@samba.org, Frank Rowand , mingo@kernel.org, Marek Szyprowski , sstabellini@kernel.org, Saravana Kannan , "list@263.net:IOMMU DRIVERS , Joerg Roedel , " , rafael.j.wysocki@intel.com, Christoph Hellwig , Bartosz Golaszewski , xen-devel@lists.xenproject.org, Thierry Reding , linux-devicetree , will@kernel.org, dan.j.williams@intel.com, linuxppc-dev@lists.ozlabs.org, Rob Herring , boris.ostrovsky@oracle.com, Andy Shevchenko , jgross@suse.com, Nicolas Boichat , Greg KH , rdunlap@infradead.org, lkml , Tomasz Figa , "list@263.net:IOMMU DRIVERS , Joerg Roedel , " , xypron.glpk@gmx.de, Robin Murphy , bauerman@linux.ibm.com Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Fri, Jan 08, 2021 at 01:39:43AM +0800, Claire Chang wrote: > On Thu, Jan 7, 2021 at 2:58 AM Konrad Rzeszutek Wilk > wrote: > > > > On Wed, Jan 06, 2021 at 11:41:23AM +0800, Claire Chang wrote: > > > Introduce the new compatible string, restricted-dma-pool, for restricted > > > DMA. One can specify the address and length of the restricted DMA memory > > > region by restricted-dma-pool in the device tree. > > > > > > Signed-off-by: Claire Chang > > > --- > > > .../reserved-memory/reserved-memory.txt | 24 +++++++++++++++++++ > > > 1 file changed, 24 insertions(+) > > > > > > diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt > > > index e8d3096d922c..44975e2a1fd2 100644 > > > --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt > > > +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt > > > @@ -51,6 +51,20 @@ compatible (optional) - standard definition > > > used as a shared pool of DMA buffers for a set of devices. It can > > > be used by an operating system to instantiate the necessary pool > > > management subsystem if necessary. > > > + - restricted-dma-pool: This indicates a region of memory meant to be > > > + used as a pool of restricted DMA buffers for a set of devices. The > > > + memory region would be the only region accessible to those devices. > > > + When using this, the no-map and reusable properties must not be set, > > > + so the operating system can create a virtual mapping that will be used > > > + for synchronization. The main purpose for restricted DMA is to > > > + mitigate the lack of DMA access control on systems without an IOMMU, > > > + which could result in the DMA accessing the system memory at > > > + unexpected times and/or unexpected addresses, possibly leading to data > > > + leakage or corruption. The feature on its own provides a basic level > > > + of protection against the DMA overwriting buffer contents at > > > + unexpected times. However, to protect against general data leakage and > > > + system memory corruption, the system needs to provide way to restrict > > > + the DMA to a predefined memory region. > > > > Heya! > > > > I think I am missing something obvious here so please bear with my > > questions: > > > > - This code adds the means of having the SWIOTLB pool tied to a specific > > memory correct? > > It doesn't affect the existing SWIOTLB. It just utilizes the existing SWIOTLB > code to create another DMA pool tied to a specific memory region for a given set > of devices. It bounces the streaming DMA (map/unmap) in and out of that region > and does the memory allocation (dma_direct_alloc) from the same region. Right, so why can't it follow the same mechanism that Xen SWIOTLB does - which had exactly the same problem (needed special handling on the pool) - and do a similar code? > > > > > > > - Nothing stops the physical device from bypassing the SWIOTLB buffer. > > That is if an errant device screwed up the length or DMA address, the > > SWIOTLB would gladly do what the device told it do? > > So the system needs to provide a way to lock down the memory access, e.g. MPU. OK! Would it be prudent to have this in the description above perhaps? > > > > > - This has to be combined with SWIOTLB-force-ish to always use the > > bounce buffer, otherwise you could still do DMA without using > > SWIOTLB (by not hitting the criteria for needing to use SWIOTLB)? > > Since restricted DMA is for the devices that are not behind an IOMMU, I change > the criteria > `if (unlikely(swiotlb_force == SWIOTLB_FORCE))` > to > `if (unlikely(swiotlb_force == SWIOTLB_FORCE) || dev->dma_io_tlb_mem)` > in dma_direct_map_page(). > > Also, even if SWIOTLB=force, the restricted DMA pool is preferred if available > (get_io_tlb_mem in https://lore.kernel.org/patchwork/patch/1360995/). > > Thanks!