[RFC] x86/tboot: add an option to disable iommu force on

[Shaohua Li]

IOMMU harms performance signficantly when we run very fast networking
workloads. This is a limitation in hardware based on our observation, so
we'd like to disable the IOMMU force on, but we do want to use TBOOT and
we can sacrifice the DMA security bought by IOMMU. I must admit I know
nothing about TBOOT, but TBOOT guys (cc-ed) think not eabling IOMMU is
totally ok.

So introduce a new boot option to disable the force on. It's kind of
silly we need to run into intel_iommu_init even without force on, but we
need to disable TBOOT PMR registers. For system without the boot option,
nothing is changed.

Signed-off-by: Shaohua Li <shli@xxxxxx>
---
 arch/x86/kernel/tboot.c       |  3 +++
 drivers/iommu/intel-iommu.c   | 21 ++++++++++++++++++++-
 include/linux/dma_remapping.h |  1 +
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index b868fa1..edbdfe6 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -510,6 +510,9 @@ int tboot_force_iommu(void)
 	if (!tboot_enabled())
 		return 0;
 
+	if (!intel_iommu_tboot_noforce)
+		return 1;
+
 	if (no_iommu || swiotlb || dmar_disabled)
 		pr_warning("Forcing Intel-IOMMU to enabled\n");
 
diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index 238ad34..737dfa7 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -183,6 +183,7 @@ static int rwbf_quirk;
  * (used when kernel is launched w/ TXT)
  */
 static int force_on = 0;
+int intel_iommu_tboot_noforce;
 
 /*
  * 0: Present
@@ -607,6 +608,10 @@ static int __init intel_iommu_setup(char *str)
 				"Intel-IOMMU: enable pre-production PASID support\n");
 			intel_iommu_pasid28 = 1;
 			iommu_identity_mapping |= IDENTMAP_GFX;
+		} else if (!strncmp(str, "tboot_noforce", 13)) {
+			 printk(KERN_INFO
+				"Intel-IOMMU: not forcing on after tboot\n");
+			intel_iommu_tboot_noforce = 1;
 		}
 
 		str += strcspn(str, ",");
@@ -4840,8 +4845,22 @@ int __init intel_iommu_init(void)
 		goto out_free_dmar;
 	}
 
-	if (no_iommu || dmar_disabled)
+	if (no_iommu || dmar_disabled) {
+		/*
+		 * We exit the function here to ensure IOMMU's remapping and
+		 * mempool aren't setup, which means that the IOMMU's PMRs
+		 * won't be disabled via the call to init_dmars(). So disable
+		 * it explicitly here. The PMRs were setup by tboot prior to
+		 * calling SENTER, but the kernel is expected to reset/tear
+		 * down the PMRs.
+		 */
+		if (intel_iommu_tboot_noforce) {
+			for_each_iommu(iommu, drhd)
+				iommu_disable_protect_mem_regions(iommu);
+		}
+
 		goto out_free_dmar;
+	}
 
 	if (list_empty(&dmar_rmrr_units))
 		pr_info("No RMRR found\n");
diff --git a/include/linux/dma_remapping.h b/include/linux/dma_remapping.h
index 187c102..9088407 100644
--- a/include/linux/dma_remapping.h
+++ b/include/linux/dma_remapping.h
@@ -39,6 +39,7 @@ extern int iommu_calculate_agaw(struct intel_iommu *iommu);
 extern int iommu_calculate_max_sagaw(struct intel_iommu *iommu);
 extern int dmar_disabled;
 extern int intel_iommu_enabled;
+extern int intel_iommu_tboot_noforce;
 #else
 static inline int iommu_calculate_agaw(struct intel_iommu *iommu)
 {
-- 
2.9.3