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


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

Michael S. Tsirkin updated VIRTIO-154:
--------------------------------------

    Proposal: 
Document VIRTIO_F_IOMMU_PLATFORM that's already
in use by Linux guests.

See message:
Date: Thu, 20 Oct 2016 18:20:01 +0100
From: Will Deacon <will.deacon@arm.com>
To: virtio-dev@lists.oasis-open.org
Cc: Will Deacon <will.deacon@arm.com>, "Michael S. Tsirkin" <mst@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>
Subject: [PATCH v2] context: Document VIRTIO_F_IOMMU_PLATFORM reserved feature bit (33)
Message-Id: <1476984001-431-1-git-send-email-will.deacon@arm.com>

https://lists.oasis-open.org/archives/virtio-dev/201610/msg00121.html


  was:
Document VIRTIO_F_IOMMU_PLATFORM that's already
in use by Linux guests.
http://markmail.org/message/b3px2pk2qyldwkwg



> 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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