x86, iommu/vt-d: Add an option to disable Intel IOMMU force on

IOMMU harms performance signficantly when we run very fast networking
workloads. It's 40GB networking doing XDP test. Software overhead is
almost unaware, but it's the IOTLB miss (based on our analysis) which
kills the performance. We observed the same performance issue even with
software passthrough (identity mapping), only the hardware passthrough
survives. The pps with iommu (with software passthrough) is only about
~30% of that without it. 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@fb.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

1 files changed, 18 insertions, 0 deletions

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index 5f08ba1..b0ced1c 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. This could expose security risk for tboot\n");
+			intel_iommu_tboot_noforce = 1;
 		}
 
 		str += strcspn(str, ",");
@@ -4851,6 +4856,19 @@ int __init intel_iommu_init(void)
 
 	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);
+		}
+
+		/*
 		 * Make sure the IOMMUs are switched off, even when we
 		 * boot into a kexec kernel and the previous kernel left
 		 * them enabled