Re: why is iommu_platform set to off by default?

[Stefan Hajnoczi]

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

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