virtio-iommu: Default to bypass during boot

virtio-iommu: Default to bypass during boot

Currently the virtio-iommu device must be programmed before it allows
DMA from any PCI device. This can make the VM entirely unusable when a
virtio-iommu driver isn't present, for example in a bootloader that
loads the OS from storage.

Similarly to the other vIOMMU implementations, default to DMA bypassing
the IOMMU during boot. Add a "boot-bypass" property, defaulting to true,
that lets users change this behavior.

Replace the VIRTIO_IOMMU_F_BYPASS feature, which didn't support bypass
before feature negotiation, with VIRTIO_IOMMU_F_BYPASS_CONFIG.

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).

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Message-Id: <20220214124356.872985-3-jean-philippe@linaro.org>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>