iommu/vt-d: Be less pessimistic about domain coherency where possible

In commit 2e12bc29 ("intel-iommu: Default to non-coherent for domains
unattached to iommus") we decided to err on the side of caution and
always assume that it's possible that a device will be attached which is
behind a non-coherent IOMMU.

In some cases, however, that just *cannot* happen. If there *are* no
IOMMUs in the system which are non-coherent, then we don't need to do
it. And flushing the dcache is a *significant* performance hit.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

1 files changed, 17 insertions, 4 deletions

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index c3d4bc91a189..1599cb1e041c 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -592,18 +592,31 @@ static struct intel_iommu *domain_get_iommu(struct dmar_domain *domain)
 
 static void domain_update_iommu_coherency(struct dmar_domain *domain)
 {
-        int i;
-
-        i = find_first_bit(domain->iommu_bmp, g_num_of_iommus);
+        struct dmar_drhd_unit *drhd;
+        struct intel_iommu *iommu;
+        int i, found = 0;
 
-        domain->iommu_coherency = i < g_num_of_iommus ? 1 : 0;
+        domain->iommu_coherency = 1;
 
         for_each_set_bit(i, domain->iommu_bmp, g_num_of_iommus) {
+                found = 1;
                 if (!ecap_coherent(g_iommus[i]->ecap)) {
                         domain->iommu_coherency = 0;
                         break;
                 }
         }
+        if (found)
+                return;
+
+        /* No hardware attached; use lowest common denominator */
+        rcu_read_lock();
+        for_each_active_iommu(iommu, drhd) {
+                if (!ecap_coherent(iommu->ecap)) {
+                        domain->iommu_coherency = 0;
+                        break;
+                }
+        }
+        rcu_read_unlock();
 }
 
 static void domain_update_iommu_snooping(struct dmar_domain *domain)