[Bug 1851664] Re: qemu-system-x86_64: "VFIO_MAP_DMA : -28" error when we attache 6 VF's to guest machine

[Alex Williamson]

Presumably w-bits (aw-bits?) implies using intel-iommu, there's a
opportunity for the vfio iommu backend to return -ENOSPC (-28) if we
exceed the default number of in-flight DMA mappings per container.  The
default limit is 65535.  You can try increasing this by changing the
dma_entry_limit module option on the vfio_iommu_type1 module.  Note that
in a typical vIOMMU config there's a container per device, so the number
of VFs attached is possibly not a factor.  It is however a lot of DMA
mappings for a single device if this is the issue and you'd generally
want to boot the guest with iommu=pt in order to have reasonable
assigned device performance with a vIOMMU, which would also greatly
reduce the number of mappings.

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https://bugs.launchpad.net/bugs/1851664

Title:
  qemu-system-x86_64: "VFIO_MAP_DMA : -28" error when we attache 6 VF's
  to guest machine

Status in QEMU:
  Incomplete

Bug description:
  We are trying to attach 6 VF's to the guest machine on 4.1.1 qemu emulator.
  We are observing "VFIO_MAP_DMA : -28" error.

  We are using w-bits=48 bits while lunching VM.

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