[PATCH v8 4/7] iommu/vt-d: Check whether device requires bounce buffer

[Lu Baolu]

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.raj@xxxxxxxxx>
Cc: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Tested-by: Xu Pengfei <pengfei.xu@xxxxxxxxx>
Tested-by: Mika Westerberg <mika.westerberg@xxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt | 5 +++++
 drivers/iommu/intel-iommu.c                     | 6 ++++++
 2 files changed, 11 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..fee5dff16e95 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 +376,8 @@ 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_untrusted(d))
+
 /*
  * Iterate over elements in device_domain_list and call the specified
  * callback @fn against each element.
@@ -457,6 +460,9 @@ 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