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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