Re: [PATCH v3 0/4] virtio-iommu: Support VIRTIO_IOMMU_F_BYPASS_CONFIG

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Feb 14 2022, Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:

> Replace the VIRTIO_IOMMU_F_BYPASS feature with
> VIRTIO_IOMMU_F_BYPASS_CONFIG, which enables a config space bit to switch
> global bypass on and off.
>
> Add a boot-bypass option, which defaults to 'on' to be in line with
> other vIOMMUs and to allow running firmware/bootloader that are unaware
> of the IOMMU. x86 doesn't need a workaround to boot with virtio-iommu
> anymore.
>
> Since v2 [1]:
> * Added the new bypass bits to the migration stream.
>   As discussed on the v2 thread, we assume that cross-version
>   compatibility is not required for live migration at the moment, so we
>   only increase the version number. Patch 2 says: "We add the bypass
>   field to the migration stream without introducing subsections, based
>   on the assumption that this virtio-iommu device isn't being used in
>   production enough to require cross-version migration at the moment
>   (all previous version required workarounds since they didn't support
>   ACPI and boot-bypass)."
>
> [1] https://lore.kernel.org/qemu-devel/20220127142940.671333-1-jean-philippe@linaro.org/

One thing that we could do to avoid surprises in the unlikely case that
somebody has a virtio-iommu device and wants to migrate to an older
machine version is to add a migration blocker for the virtio-iommu
device for all compat machines for versions 6.2 or older (i.e. only 7.0
or newer machine types can have a migratable virtio-iommu device
starting with QEMU 7.0.) Not too complicated to implement, but I'm not
sure whether we'd add too much code to prevent something very unlikely
to happen anyway. I would not insist on it :)

Otherwise, I didn't spot problems in this series.