From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [Qemu-devel] [PATCH 2/6] PCI DMA API Date: Sun, 30 Mar 2008 14:02:25 -0500 Message-ID: <47EFE3C1.1000205@us.ibm.com> References: <1206827760-4566-1-git-send-email-aliguori@us.ibm.com> <200803301118.15663.paul@codesourcery.com> <47EFA6E1.7020401@us.ibm.com> <200803301919.54780.paul@codesourcery.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel@lists.sourceforge.net, Marcelo Tosatti , qemu-devel@nongnu.org, Aurelien Jarno To: Paul Brook Return-path: In-Reply-To: <200803301919.54780.paul@codesourcery.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces@lists.sourceforge.net Errors-To: kvm-devel-bounces@lists.sourceforge.net List-Id: kvm.vger.kernel.org Paul Brook wrote: > On Sunday 30 March 2008, Anthony Liguori wrote: > > The entity processing the data shouldn't need to know or care how the > translation is done. PhysIOVector should describe everything it need to know. > Okay, I'll update. >> What could work is if the DMA API functions mapped PhysIOVector => >> PhysIOVector and then the network and block subsystems could operate on >> a PhysIOVector. I have patches that implement vector IO for net and >> block but didn't want to include them in this series to keep things simple. >> > > IMHO this is the only sane way to implement zero-copy. > > >>>> This enables zero-copy IO to be preformed without introducing >>>> assumptions of phys_ram_base. This API is at the PCI device level to >>>> enable support of per-device IOMMU remapping. >>>> >>> By my reading it *requires* bridges be zero-copy. For big-endian targets >>> we need to ability to byteswap accesses. >>> >> You mean via ld/st_phys? >> > > By whatever means the bridge deems necessary. The whole point of the DMA API > is that you're transferring a block of data. The API allows intermediate > busses to transform that data (and address) without the block handler needing > to know or care. > > With your current scheme a byteswapping bus has to allocate a single large > buffer for the whole vector, even if the device then ends up copying unto a > local buffer in small chunks. > Oh, I see now. The DMA API should have not just a mechanism to do bulk transfers but also provide an interface to do load/store's that could potentially be byte-swapped. I didn't realize buses did that. Regards, Anthony Liguori > Paul > ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://ad.doubleclick.net/clk;164216239;13503038;w?http://sf.net/marketplace