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    SubjectRe: [RFC V2] virtio: Add platform specific DMA API translation for virito devices
    On Thu, May 24, 2018 at 08:27:04AM +1000, Benjamin Herrenschmidt wrote:
    > On Wed, 2018-05-23 at 21:50 +0300, Michael S. Tsirkin wrote:
    >
    > > I re-read that discussion and I'm still unclear on the
    > > original question, since I got several apparently
    > > conflicting answers.
    > >
    > > I asked:
    > >
    > > Why isn't setting VIRTIO_F_IOMMU_PLATFORM on the
    > > hypervisor side sufficient?
    >
    > I thought I had replied to this...
    >
    > There are a couple of reasons:
    >
    > - First qemu doesn't know that the guest will switch to "secure mode"
    > in advance. There is no difference between a normal and a secure
    > partition until the partition does the magic UV call to "enter secure
    > mode" and qemu doesn't see any of it. So who can set the flag here ?

    This seems weird to me. As a rule HV calls should go through qemu -
    or be allowed to go directly to KVM *by* qemu. We generally reserve
    the latter for hot path things. Since this isn't a hot path, having
    the call handled directly by the kernel seems wrong.

    Unless a "UV call" is something different I don't know about.

    > - Second, when using VIRTIO_F_IOMMU_PLATFORM, we also make qemu (or
    > vhost) go through the emulated MMIO for every access to the guest,
    > which adds additional overhead.
    >

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