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    SubjectRe: [PATCH 1/2] IOMMU Identity Mapping Support: iommu_identity_mapping definition
    * Muli Ben-Yehuda (muli@il.ibm.com) wrote:
    > On Thu, Jun 18, 2009 at 11:14:51AM -0700, Yu, Fenghua wrote:
    > > >
    > > >On Thu, Jun 18, 2009 at 11:05:14AM -0700, Fenghua Yu wrote:
    > > >
    > > >> IOMMU Identity Mapping Support: iommu_identity_mapping definition
    > > >>
    > > >> Identity mapping for IOMMU defines a single domain to 1:1 map all
    > > >> pci devices to all usable memory.
    > > >
    > > >Why use VT-d at all in this case? Do you have a use-case in mind?
    > >
    > > Some users want to use VT-d in KVM but are concerned of DMA
    > > remapping performance. They can use identity mapping and still have
    > > KVM on VT-d. They can also use pass through patch (sent out before)
    > > if hardware supports pass through.
    >
    > Sorry, I must be missing something. For the normal device assignment
    > case, we want the IOMMU page tables to have gpa->hpa mappings rather
    > than the 1-1 identity mapping. How do you envision the 1-1 mapping
    > being used in the device assignment case?

    The 1-1 mapping is for all the host devices _not_ assigned to guests.
    To eliminate the i/o overhead imposed on all guests not using an
    assigned device or from i/o from host.

    It's just the same as VT-d PassThrough mode for hardware that doesn't
    support it.

    thanks,
    -chris


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