[PATCH 0/5] iommu/virtio: Add identity domains

Tian, Kevin
2021-10-14T03:00:38Z

> From: Jean-Philippe Brucker <jean-philippe at linaro.org>
> Sent: Wednesday, October 13, 2021 8:11 PM
>
> Support identity domains, allowing to only enable IOMMU protection for a
> subset of endpoints (those assigned to userspace, for example). Users
> may enable identity domains at compile time
> (CONFIG_IOMMU_DEFAULT_PASSTHROUGH), boot time
> (iommu.passthrough=1) or
> runtime (/sys/kernel/iommu_groups/*/type = identity).

Do we want to use consistent terms between spec (bypass domain)
and code (identity domain)?

>
> Patches 1-2 support identity domains using the optional
> VIRTIO_IOMMU_F_BYPASS_CONFIG feature. The feature bit is not yet in the
> spec, see [1] for the latest proposal.
>
> Patches 3-5 add a fallback to identity mappings, when the feature is not
> supported.
>
> Note that this series doesn't touch the global bypass bit added by
> VIRTIO_IOMMU_F_BYPASS_CONFIG. All endpoints managed by the IOMMU
> should
> be attached to a domain, so global bypass isn't in use after endpoints

I saw a concept of deferred attach in iommu core. See iommu_is_
attach_deferred(). Currently this is vendor specific and I haven't
looked into the exact reason why some vendor sets it now. Just
be curious whether the same reason might be applied to virtio-iommu.

> are probed. Before that, the global bypass policy is decided by the
> hypervisor and firmware. So I don't think Linux needs to touch the

This reminds me one thing. The spec says that the global bypass
bit is sticky and not affected by reset. This implies that in the case
of rebooting the VM into a different OS, the previous OS actually
has the right to override this setting for the next OS. Is it a right
design? Even the firmware itself is unable to identify the original
setting enforced by the hypervisor after reboot. I feel the hypervisor
setting should be recovered after reset since it reflects the
security measure enforced by the virtual platform?

> global bypass bit, but there are some patches available on my
> virtio-iommu/bypass branch [2] to test it.
>
> QEMU patches are on my virtio-iommu/bypass branch [3] (and the list)
>
> [1] https://www.mail-archive.com/virtio-dev at lists.oasis-
> open.org/msg07898.html
> [2] https://jpbrucker.net/git/linux/log/?h=virtio-iommu/bypass
> [3] https://jpbrucker.net/git/qemu/log/?h=virtio-iommu/bypass
>
> Jean-Philippe Brucker (5):
>   iommu/virtio: Add definitions for VIRTIO_IOMMU_F_BYPASS_CONFIG
>   iommu/virtio: Support bypass domains
>   iommu/virtio: Sort reserved regions
>   iommu/virtio: Pass end address to viommu_add_mapping()
>   iommu/virtio: Support identity-mapped domains
>
>  include/uapi/linux/virtio_iommu.h |   8 ++-
>  drivers/iommu/virtio-iommu.c      | 113 +++++++++++++++++++++++++-----
>  2 files changed, 101 insertions(+), 20 deletions(-)
>
> --
> 2.33.0