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>

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index c3d4bc91a189a7c34ef7f1809fa0feb474cad1a9..1599cb1e041c5f28462f4d96266444905e5fa86c 100644 (file)
--- 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)