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    SubjectRe: [PATCH]intel-iommu batched iotlb flushes
    On Tue, Feb 12, 2008 at 01:00:06AM -0800, David Miller wrote:
    > From: Muli Ben-Yehuda <muli@il.ibm.com>
    > 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



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