[PATCH] intel-iommu: Default to non-coherent for domains unattached to iommus

[Donald Dutile <ddutile-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

This patch was posted back in Nov 2011:
  http://lists.linuxfoundation.org/pipermail/iommu/2011-November/003086.html

and due to discussion about the patch, it was never pulled in.
Although the thread discussed an alternate patch to
default to non-coherent if any IOMMU didn't support coherency,
this alternate method was never implemented, and this bug persists.

This patch has been in RHEL6 for quite some time,
and it wasn't noticed that it didn't get into linux upstream, 
until a RH partner reported this error when running upstream kernels,
and noticed how it doesn't occur on RHEL6 kernels.
Applying this patch to an upstream kernel resolved this issue.


 domain_update_iommu_coherency() currently defaults to setting
 domains as coherent when the domain is not attached to any iommus.
 This allows for a window in domain_context_mapping_one() where such a
 domain can update context entries non-coherently, and only after
 update the domain capability to clear iommu_coherency.
 This can be seen using KVM device assignment on VT-d systems that
 do not support coherency in the ecap register.  When a device is
 added to a guest, a domain is created (iommu_coherency = 0), the
 device is attached, and ranges are mapped.  If we then hot unplug
 the device, the coherency is updated and set to the default (1)
 since no iommus are attached to the domain.  A subsequent attach
 of a device makes use of the same dmar domain (now marked coherent)
 updates context entries with coherency enabled, and only disables
 coherency as the last step in the process.
 To fix this, switch domain_update_iommu_coherency() to use the
 safer, non-coherent default for domains not attached to iommus.

Signed-off-by: Donald Dutile <ddutile-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
cc: Alex Williamson <alex.williamson at redhat.com>
---
 drivers/iommu/intel-iommu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index 2297ec1..52ff63f 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -589,7 +589,9 @@ static void domain_update_iommu_coherency(struct dmar_domain *domain)
 {
 	int i;
 
-	domain->iommu_coherency = 1;
+	i = find_first_bit(domain->iommu_bmp, g_num_of_iommus);
+
+	domain->iommu_coherency = i < g_num_of_iommus ? 1: 0;
 
 	for_each_set_bit(i, domain->iommu_bmp, g_num_of_iommus) {
 		if (!ecap_coherent(g_iommus[i]->ecap)) {
-- 
1.7.10.2.552.gaa3bb87