On Tue, Feb 12, 2008 at 01:00:06AM -0800, David Miller wrote: > From: Muli Ben-Yehuda <[EMAIL PROTECTED]> > Date: Tue, 12 Feb 2008 10:52:56 +0200 > > > The streaming DMA-API was designed to conserve IOMMU mappings for > > machines where IOMMU mappings are a scarce resource, and is a poor > > fit for a modern IOMMU such as VT-d with a 64-bit IO address space > > (or even an IOMMU with a 32-bit address space such as Calgary) > > where there are plenty of IOMMU mappings available. > > For the 64-bit case what you are suggesting eventually amounts to > mapping all available RAM in the IOMMU. > > Although an extreme version of your suggestion, it would be the most > efficient as it would require zero IOMMU flush operations. > > But we'd lose things like protection and other benefits.
For the extreme case you are correct. There's an inherent trade-off between IOMMU performance and protection guarantees, where one end of the spectrum is represented by the streaming DMA-API and the other end is represented by simply mapping all available memory. It's an open question what is the right point in between. I think that an optimal strategy might be "keep the mapping around for as long as it is safe", i.e., keep a mapping to a frame for as long as the frame is owned by whoever requested the mapping in the first place. Once ownership of the frame is passed to another entity (e.g., from the driver to the stack), revoke the original mapping. This implies a way for the kernel to track frame ownership and communicate frame ownership changes to the DMA-API layer, which we don't currently have. Cheers, Muli -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
