[PATCH kernel v3] vfio/spapr: Add cond_resched() for huge updates

[Alexey Kardashevskiy <aik@ozlabs.ru>]

Clearing very big IOMMU tables can trigger soft lockups. This adds
cond_resched() to allow the scheduler to do context switching when
it decides to.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---

The testcase is POWER9 box with 264GB guest, 4 VFIO devices from
independent IOMMU groups, 64K IOMMU pages. This configuration produces
4325376 TCE entries, each entry update incurs 4 OPAL calls to update
an individual PE TCE cache; this produced lockups for more than 20s.
Reducing table size to 4194304 (i.e. 256GB guest) or removing one
of 4 VFIO devices makes the problem go away.

---
Changes:
v3:
* cond_resched() checks for should_resched() so we just call resched()
and let the cpu scheduler decide whether to switch or not

v2:
* replaced with time based solution
---
 drivers/vfio/vfio_iommu_spapr_tce.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
index 63112c36ab2d..759a5bdd40e1 100644
--- a/drivers/vfio/vfio_iommu_spapr_tce.c
+++ b/drivers/vfio/vfio_iommu_spapr_tce.c
@@ -507,6 +507,8 @@ static int tce_iommu_clear(struct tce_container *container,
 	enum dma_data_direction direction;
 
 	for ( ; pages; --pages, ++entry) {
+		cond_resched();
+
 		direction = DMA_NONE;
 		oldhpa = 0;
 		ret = iommu_tce_xchg(tbl, entry, &oldhpa, &direction);
-- 
2.11.0