[PATCH v3 2/4] iommu: Add capability for pre-boot DMA protection

[Robin Murphy <robin.murphy@arm.com>]

VT-d's dmar_platform_optin() actually represents a combination of
properties fairly well standardised by Microsoft as "Pre-boot DMA
Protection" and "Kernel DMA Protection"[1]. As such, we can provide
interested consumers with an abstracted capability rather than
driver-specific interfaces that won't scale. We name it for the former
aspect since that's what external callers are most likely to be
interested in; the latter is for the IOMMU layer to handle itself.

[1] https://docs.microsoft.com/en-us/windows-hardware/design/device-experiences/oem-kernel-dma-protection

Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
 drivers/iommu/intel/iommu.c | 2 ++
 include/linux/iommu.h       | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 255304eb3b1f..49d552a96098 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -4550,6 +4550,8 @@ static bool intel_iommu_capable(struct device *dev, enum iommu_cap cap)
 		return domain_update_iommu_snooping(NULL);
 	if (cap == IOMMU_CAP_INTR_REMAP)
 		return irq_remapping_enabled == 1;
+	if (cap == IOMMU_CAP_PRE_BOOT_PROTECTION)
+		return dmar_platform_optin();
 
 	return false;
 }
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 1fa927e6f1c6..64c02f472f7b 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -107,6 +107,8 @@ enum iommu_cap {
 					   transactions */
 	IOMMU_CAP_INTR_REMAP,		/* IOMMU supports interrupt isolation */
 	IOMMU_CAP_NOEXEC,		/* IOMMU_NOEXEC flag */
+	IOMMU_CAP_PRE_BOOT_PROTECTION,	/* Firmware says it used the IOMMU for
+					   DMA protection and we should too */
 };
 
 /* These are the possible reserved region types */
-- 
2.28.0.dirty