Re: why is iommu_platform set to off by default?

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 30, 2021 at 04:38:11PM +0000, Stefan Hajnoczi wrote:
> On Tue, Nov 30, 2021 at 02:32:49PM +0000, Peter Maydell wrote:
> > I've just spent a day or so trying to track down why PCI passthrough
> > of a virtio-blk-pci device wasn't working. The problem turns out to be
> > that by default virtio pci devices don't use the IOMMU, even when the
> > machine model has created an IOMMU and arranged for the PCI bus to
> > be underneath it. So when the L2 guest tries to program the virtio device,
> > the virtio device treats the IPAs it writes as if they were PAs and
> > of course the data structures it's looking for aren't there.
> > 
> > Why do we default this to 'off'? It seems pretty unhelpful not to
> > honour the existence of the IOMMU, and the failure mode is pretty
> > opaque (L2 guest just hangs)...
> 
> Historically VIRTIO used guest physical addresses instead of bus
> addresses (IOVA). I think when IOMMU support was added, a QEMU -device
> virtio-* parameter was added and it's simply off by default:
> 
>   iommu_platform=<bool>  - on/off (default: false)
> 
> Maybe this behavior is for backwards compatibility? Existing guests with
> IOMMUs shouldn't change behavior, although this could be fixed with
> machine type compat properties.
> 
> Stefan

Unfortunately iommu is special. Bypassing it makes the config
insecure for nested virt, so it's important that we do not
allow guest driver to disable the iommu through the
feature bit.
This means normal compat machinery (make feature on by default
but disable for legacy drivers) does not work here.

-- 
MST