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    SubjectRe: [PATCH 3/5] vDPA: introduce vDPA bus
    On Mon, Jan 20, 2020 at 05:49:39PM +0000, Jason Gunthorpe wrote:
    > On Mon, Jan 20, 2020 at 04:43:53PM +0800, Jason Wang wrote:
    > > This is similar to the design of platform IOMMU part of vhost-vdpa. We
    > > decide to send diffs to platform IOMMU there. If it's ok to do that in
    > > driver, we can replace set_map with incremental API like map()/unmap().
    > >
    > > Then driver need to maintain rbtree itself.
    >
    > I think we really need to see two modes, one where there is a fixed
    > translation without dynamic vIOMMU driven changes and one that
    > supports vIOMMU.
    >
    > There are different optimization goals in the drivers for these two
    > configurations.
    >
    > > > If the first one, then I think memory hotplug is a heavy flow
    > > > regardless. Do you think the extra cycles for the tree traverse
    > > > will be visible in any way?
    > >
    > > I think if the driver can pause the DMA during the time for setting up new
    > > mapping, it should be fine.
    >
    > This is very tricky for any driver if the mapping change hits the
    > virtio rings. :(
    >
    > Even a IOMMU using driver is going to have problems with that..
    >
    > Jason

    I think for starters we can assume this doesn't happen,
    so any change doesn't affect any buffers in use.
    Certainly true e.g. for memory hotplug.

    --
    MST

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