On Sunday 30 March 2008, Anthony Liguori wrote: > Paul Brook wrote: > > On Saturday 29 March 2008, Anthony Liguori wrote: > >> This patch introduces a PCI DMA API and some generic code to support > >> other DMA APIs. Two types are introduced: PhysIOVector and IOVector. A > >> DMA API maps a PhysIOVector, which is composed of target_phys_addr_t, > >> into an IOVector, which is composed of void *. > > > > Devices should not be using IOVector. They should either use the DMA copy > > routines to copy from a PhysIOVector into a local buffer, or they should > > pass a PhysIOVector to a block/network read/write routine. The DMA API > > should allow devices to be agnostic about how DMA is implemented. They > > should not be trying to manually implement zero copy. > > Someone has to do the translation of PhysIOVector => IOVector. It > doesn't seem logical to me to do it in the IO backend level because the > block subsystem doesn't know how to do that translation. You would have > to pass the PhysIOVector although with a translation function and an > opaque pointer.
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. > 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. 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 _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
