[PATCH v2] vfio iommu: Add dma available capability

[Matthew Rosato]

Commit 492855939bdb ("vfio/type1: Limit DMA mappings per container") added
a limit to the number of concurrent DMA requests for a vfio container.
However, lazy unmapping in s390 can in fact cause quite a large number of
outstanding DMA requests to build up prior to being purged, potentially 
the entire guest DMA space.  This results in unexpected 'VFIO_MAP_DMA
failed: No space left on device' conditions seen in QEMU.

This patch proposes to provide the remaining number of allowable DMA 
requests via the VFIO_IOMMU_GET_INFO ioctl as a new capability.  A 
subsequent patchset to QEMU would collect this information and use it in 
s390 PCI support to tap the guest on the shoulder before overrunning the 
vfio limit.

Changes from v1:
- Report dma_avail instead of the limit, which might not have been accurate
  anyway
- Text/naming changes throughout due to the above

Matthew Rosato (1):
  vfio iommu: Add dma available capability

 drivers/vfio/vfio_iommu_type1.c | 17 +++++++++++++++++
 include/uapi/linux/vfio.h       | 16 ++++++++++++++++
 2 files changed, 33 insertions(+)

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1.8.3.1