[PATCH v12 4/6] iommu/s390: Disable deferred flush for ISM devices

Niklas Schnelle
2023-08-25T10:14:04Z

ISM devices are virtual PCI devices used for cross-LPAR communication.
Unlike real PCI devices ISM devices do not use the hardware IOMMU but
inspects IOMMU translation tables directly on IOTLB flush (s390 RPCIT
instruction).

ISM devices keep their DMA allocations static and only very rarely DMA
unmap at all. For each IOTLB flush that occurs after unmap the ISM
devices will however inspect the area of the IOVA space indicated by the
flush. This means that for the global IOTLB flushes used by the flush
queue mechanism the entire IOVA space would be inspected. In principle
this would be fine, albeit potentially unnecessarily slow, it turns out
however that ISM devices are sensitive to seeing IOVA addresses that are
currently in use in the IOVA range being flushed. Seeing such in-use
IOVA addresses will cause the ISM device to enter an error state and
become unusable.

Fix this by claiming IOMMU_CAP_DEFERRED_FLUSH only for non-ISM devices.
This makes sure IOTLB flushes only cover IOVAs that have been unmapped
and also restricts the range of the IOTLB flush potentially reducing
latency spikes.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
drivers/iommu/s390-iommu.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
index f6d6c60e5634..8310180a102c 100644
--- a/drivers/iommu/s390-iommu.c
+++ b/drivers/iommu/s390-iommu.c
@@ -315,11 +315,13 @@ static struct s390_domain *to_s390_domain(struct iommu_domain *dom)

static bool s390_iommu_capable(struct device *dev, enum iommu_cap cap)
{
+ struct zpci_dev *zdev = to_zpci_dev(dev);
+
switch (cap) {
case IOMMU_CAP_CACHE_COHERENCY:
return true;
case IOMMU_CAP_DEFERRED_FLUSH:
- return true;
+ return zdev->pft != PCI_FUNC_TYPE_ISM;
default:
return false;
}

--
2.39.2