This adds a helper to check whether a device needs to
use bounce buffer. It also provides a boot time option
to disable the bounce buffer. Users can use this to
prevent the iommu driver from using the bounce buffer
for performance gain.

Cc: Ashok Raj <ashok....@intel.com>
Cc: Jacob Pan <jacob.jun....@linux.intel.com>
Cc: Kevin Tian <kevin.t...@intel.com>
Signed-off-by: Lu Baolu <baolu...@linux.intel.com>
Tested-by: Xu Pengfei <pengfei...@intel.com>
Tested-by: Mika Westerberg <mika.westerb...@intel.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 5 +++++
 drivers/iommu/intel-iommu.c                     | 7 +++++++
 2 files changed, 12 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 4c1971960afa..f441a9cea5bb 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1732,6 +1732,11 @@
                        Note that using this option lowers the security
                        provided by tboot because it makes the system
                        vulnerable to DMA attacks.
+               nobounce [Default off]
+                       Disable bounce buffer for unstrusted devices such as
+                       the Thunderbolt devices. This will treat the untrusted
+                       devices as the trusted ones, hence might expose security
+                       risks of DMA attacks.
 
        intel_idle.max_cstate=  [KNL,HW,ACPI,X86]
                        0       disables intel_idle and fall back on acpi_idle.
diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index c4e0e4a9ee9e..e12aa73008df 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -362,6 +362,7 @@ static int dmar_forcedac;
 static int intel_iommu_strict;
 static int intel_iommu_superpage = 1;
 static int iommu_identity_mapping;
+static int intel_no_bounce;
 
 #define IDENTMAP_ALL           1
 #define IDENTMAP_GFX           2
@@ -375,6 +376,9 @@ EXPORT_SYMBOL_GPL(intel_iommu_gfx_mapped);
 static DEFINE_SPINLOCK(device_domain_lock);
 static LIST_HEAD(device_domain_list);
 
+#define device_needs_bounce(d) (!intel_no_bounce && dev_is_pci(d) &&   \
+                               to_pci_dev(d)->untrusted)
+
 /*
  * Iterate over elements in device_domain_list and call the specified
  * callback @fn against each element.
@@ -457,6 +461,9 @@ static int __init intel_iommu_setup(char *str)
                        printk(KERN_INFO
                                "Intel-IOMMU: not forcing on after tboot. This 
could expose security risk for tboot\n");
                        intel_iommu_tboot_noforce = 1;
+               } else if (!strncmp(str, "nobounce", 8)) {
+                       pr_info("Intel-IOMMU: No bounce buffer. This could 
expose security risks of DMA attacks\n");
+                       intel_no_bounce = 1;
                }
 
                str += strcspn(str, ",");
-- 
2.17.1

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