[OASIS Issue Tracker] (VIRTIO-154) Document VIRTIO_F_IOMMU_PLATFORM reserved feature bit (33)

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    [ https://issues.oasis-open.org/browse/VIRTIO-154?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=64261#comment-64261 ] 

Michael S. Tsirkin commented on VIRTIO-154:
-------------------------------------------

BALLOT CREATED AT URL: https://www.oasis-open.org/apps/org/workgroup/virtio/ballot.php?id=3012

> Document VIRTIO_F_IOMMU_PLATFORM reserved feature bit (33)
> ----------------------------------------------------------
>
>                 Key: VIRTIO-154
>                 URL: https://issues.oasis-open.org/browse/VIRTIO-154
>             Project: OASIS Virtual I/O Device (VIRTIO) TC
>          Issue Type: Bug
>    Affects Versions: virtio 1.0 cs04
>         Environment: Reported-by: Will Deacon <will.deacon@arm.com>
>            Reporter: Michael S. Tsirkin
>            Assignee: Michael S. Tsirkin
>             Fix For: virtio 1.1 cs01
>
>
> On most virtio systems, physical addresses are provided from the driver
> to the device and subsequently used to access memory directly. However,
> some systems feature an IOMMU that complicates things slightly:
>   - The IOMMU might require configuration for DMA to work at all. Once
>     configured, the driver must pass bus addresses to the device, which
>     are then translated by the IOMMU into physical addresses in memory.
>   - The IOMMU might be bypassed by virtio DMA. In this case, physical
>     addresses must still be provided to the device by the driver, even
>     though an IOMMU appears to be present.
> This has been solved by the addition of a new reserved feature bit,
> VIRTIO_F_IOMMU_PLATFORM, to describe the two cases above. Legacy systems
> will not negotiate the feature, and therefore continue to operate
> directly on physical addresses.



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