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Subject[PATCH 3/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency
From: Jason Gunthorpe <>

The KVM mechanism for controlling wbinvd is only triggered during
kvm_vfio_group_add(), meaning it is a one-shot test done once the devices
are setup.

So, there is no value in trying to push a device that could do enforced
cache coherency to a dedicated domain vs re-using an existing domain since
KVM won't be able to take advantage of it. This just wastes domain memory.

Simplify this code and eliminate the test. This removes the only logic
that needed to have a dummy domain attached prior to searching for a
matching domain and simplifies the next patches.

If someday we want to try and optimize this further the better approach is
to update the Intel driver so that enforce_cache_coherency() can work on a
domain that already has IOPTEs and then call the enforce_cache_coherency()
after detaching a device from a domain to upgrade the whole domain to
enforced cache coherency mode.

Signed-off-by: Jason Gunthorpe <>
Signed-off-by: Nicolin Chen <>
drivers/vfio/vfio_iommu_type1.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index c13b9290e357..f4e3b423a453 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -2285,9 +2285,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
* testing if they're on the same bus_type.
list_for_each_entry(d, &iommu->domain_list, next) {
- if (d->domain->ops == domain->domain->ops &&
- d->enforce_cache_coherency ==
- domain->enforce_cache_coherency) {
+ if (d->domain->ops == domain->domain->ops) {
iommu_detach_group(domain->domain, group->iommu_group);
if (!iommu_attach_group(d->domain,
group->iommu_group)) {
 \ /
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