In [1] we attempted to fix a case where a VFIO-PCI device protected with a virtio-iommu was assigned to an x86 guest. On x86 the physical IOMMU may have an address width (gaw) of 39 or 48 bits whereas the virtio-iommu used to expose a 64b address space by default. Hence the guest was trying to use the full 64b space and we hit DMA MAP failures. To work around this issue we managed to pass usable IOVA regions (excluding the out of range space) from VFIO to the virtio-iommu device. This was made feasible by introducing a new IOMMU Memory Region callback dubbed iommu_set_iova_regions(). This latter gets called when the IOMMU MR is enabled which causes the vfio_listener_region_add() to be called.
However with VFIO-PCI hotplug, this technique fails due to the race between the call to the callback in the add memory listener and the virtio-iommu probe request. Indeed the probe request gets called before the attach to the domain. So in that case the usable regions are communicated after the probe request and fail to be conveyed to the guest. To be honest the problem was hinted by Jean-Philippe in [1] and I should have been more careful at listening to him and testing with hotplug :-( For coldplugged device the technique works because we make sure all the IOMMU MR are enabled once on the machine init done: 94df5b2180 ("virtio-iommu: Fix 64kB host page size VFIO device assignment") for granule freeze. But I would be keen to get rid of this trick. Using an IOMMU MR Ops is unpractical because this relies on the IOMMU MR to have been enabled and the corresponding vfio_listener_region_add() to be executed. Instead this series proposes to replace the usage of this API by the recently introduced PCIIOMMUOps: ba7d12eb8c ("hw/pci: modify pci_setup_iommu() to set PCIIOMMUOps"). That way, the callback can be called earlier, once the usable IOVA regions have been collected by VFIO, without the need for the IOMMU MR to be enabled. This looks cleaner. In the short term this may also be used for passing the page size mask, which would allow to get rid of the hacky transient IOMMU MR enablement mentionned above. [1] [PATCH v4 00/12] VIRTIO-IOMMU/VFIO: Don't assume 64b IOVA space https://lore.kernel.org/all/20231019134651.842175-1-eric.au...@redhat.com/ [2] https://lore.kernel.org/all/20230929161547.GB2957297@myrica/ Extra Notes: With that series, the reserved memory regions are communicated on time so that the virtio-iommu probe request grabs them. However this is not sufficient. In some cases (my case), I still see some DMA MAP failures and the guest keeps on using IOVA ranges outside the geometry of the physical IOMMU. This is due to the fact the VFIO-PCI device is in the same iommu group as the pcie root port. Normally the kernel iova_reserve_iommu_regions (dma-iommu.c) is supposed to call reserve_iova() for each reserved IOVA, which carves them out of the allocator. When iommu_dma_init_domain() gets called for the hotplugged vfio-pci device the iova domain is already allocated and set and we don't call iova_reserve_iommu_regions() again for the vfio-pci device. So its corresponding reserved regions are not properly taken into account. This is not trivial to fix because theoretically the 1st attached devices could already have allocated IOVAs within the reserved regions of the second device. Also we are somehow hijacking the reserved memory regions to model the geometry of the physical IOMMU so not sure any attempt to fix that upstream will be accepted. At the moment one solution is to make sure assigned devices end up in singleton group. Another solution is to work on a different approach where the gaw can be passed as an option to the virtio-iommu device, similarly at what is done with intel iommu. This series can be found at: https://github.com/eauger/qemu/tree/hotplug-resv-rfc Eric Auger (7): hw/pci: Introduce PCIIOMMUOps::set_host_iova_regions hw/pci: Introduce pci_device_iommu_bus vfio/pci: Pass the usable IOVA ranges through PCIIOMMUOps virtio-iommu: Implement PCIIOMMUOps set_host_resv_regions virtio-iommu: Remove the implementation of iommu_set_iova_ranges hw/vfio: Remove memory_region_iommu_set_iova_ranges() call memory: Remove IOMMU MR iommu_set_iova_range API include/exec/memory.h | 32 ------- include/hw/pci/pci.h | 16 ++++ hw/pci/pci.c | 16 ++++ hw/vfio/common.c | 10 -- hw/vfio/pci.c | 27 ++++++ hw/virtio/virtio-iommu.c | 201 ++++++++++++++++++++------------------- system/memory.c | 13 --- 7 files changed, 160 insertions(+), 155 deletions(-) -- 2.41.0