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Subject[PATCH v4 6/9] iommu/vt-d: Check whether device requires bounce buffer
Date
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@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Tested-by: Xu Pengfei <pengfei.xu@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@intel.com>
---
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 138f6664b2e2..65685c6e53e4 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1728,6 +1728,11 @@
Note that using this option lowers the security
provided by tboot because it makes the system
vulnerable to DMA attacks.
+ nobounce [Default off]
+ Do not use the bounce buffer for untrusted devices like
+ 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 235837c50719..41439647f75d 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -371,6 +371,7 @@ 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
@@ -384,6 +385,8 @@ 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.
@@ -466,6 +469,9 @@ 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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