Hi Eric,
>-----Original Message-----
>From: Eric Auger <eric.au...@redhat.com>
>Cc: m...@redhat.com; c...@redhat.com
>Subject: [RFC 0/7] VIRTIO-IOMMU/VFIO: Fix host iommu geometry handling
>for hotplugged devices
>
>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 :-(
It looks the global virtio_iommu_config.bypass is never cleared in guest.
When guest virtio_iommu driver enable IOMMU, should it clear this
bypass attribute?
If it could be cleared in viommu_probe(), then qemu will call
virtio_iommu_set_config() then virtio_iommu_switch_address_space_all()
to enable IOMMU MR. Then both coldplugged and hotplugged devices will work.
Intel iommu has a similar bit in register GCMD_REG.TE, when guest
intel_iommu driver probe set it, on qemu side, vtd_address_space_refresh_all()
is called to enable IOMMU MRs.
>
>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.
I suspect there is same issue with coldplugged devices. If those devices
are in same group, get iova_reserve_iommu_regions() is only called
for first device. But other devices's reserved regions are missed.
Curious how you make passthrough device and pcie root port under same
group.
When I start a x86 guest with passthrough device, I see passthrough
device and pcie root port are in different group.
-[0000:00]-+-00.0
+-01.0
+-02.0
+-03.0-[01]----00.0
/sys/kernel/iommu_groups/3/devices:
0000:00:03.0
/sys/kernel/iommu_groups/7/devices:
0000:01:00.0
My qemu cmdline:
-device pcie-root-port,id=root0,slot=0
-device vfio-pci,host=6f:01.0,id=vfio0,bus=root0
Thanks
Zhenzhong
>
>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
