[PATCH v3 1/2] iommupt: Do not set C-bit on MMIO backed PTEs

[Wei Wang]

AMD Secure Memory Encryption (SME) marks individual memory pages as
encrypted by setting the C-bit in page table entries. According to the
AMD APM,any pages corresponding to MMIO addresses must be configured
with the C-bit clear.

The current *_iommu_set_prot() implementation sets the C-bit on all PTEs
in the IOMMU page tables. This is incorrect for PTEs backed by MMIO, and
can break PCIe peer-to-peer communication when IOVA is used. Fix this by
avoiding the C-bit for MMIO-backed mappings.

For amdv2 IOMMU page tables, there is a usage scenario for GVA->GPA
mappings, and for the trusted MMIO in the TEE-IO case, the C-bit will need
to be added to GPA. However, SNP guests do not yet support vIOMMU, and the
trusted MMIO support is not ready in upstream. Adding the C-bit for trusted
MMIO can be considered once those features land.

Fixes: 879ced2bab1b ("iommupt: Add the AMD IOMMU v1 page table format")
Fixes: aef5de756ea8 ("iommupt: Add the x86 64 bit page table format")
Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxxxx>
---
 drivers/iommu/generic_pt/fmt/amdv1.h  | 3 ++-
 drivers/iommu/generic_pt/fmt/x86_64.h | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/generic_pt/fmt/amdv1.h b/drivers/iommu/generic_pt/fmt/amdv1.h
index aa8e1a8ec95f..3b2c41d9654d 100644
--- a/drivers/iommu/generic_pt/fmt/amdv1.h
+++ b/drivers/iommu/generic_pt/fmt/amdv1.h
@@ -354,7 +354,8 @@ static inline int amdv1pt_iommu_set_prot(struct pt_common *common,
 	 * Ideally we'd have an IOMMU_ENCRYPTED flag set by higher levels to
 	 * control this. For now if the tables use sme_set then so do the ptes.
 	 */
-	if (pt_feature(common, PT_FEAT_AMDV1_ENCRYPT_TABLES))
+	if (pt_feature(common, PT_FEAT_AMDV1_ENCRYPT_TABLES) &&
+	    !(iommu_prot & IOMMU_MMIO))
 		pte = __sme_set(pte);
 
 	attrs->descriptor_bits = pte;
diff --git a/drivers/iommu/generic_pt/fmt/x86_64.h b/drivers/iommu/generic_pt/fmt/x86_64.h
index a86353f1481e..ec2143488e8b 100644
--- a/drivers/iommu/generic_pt/fmt/x86_64.h
+++ b/drivers/iommu/generic_pt/fmt/x86_64.h
@@ -227,7 +227,8 @@ static inline int x86_64_pt_iommu_set_prot(struct pt_common *common,
 	 * Ideally we'd have an IOMMU_ENCRYPTED flag set by higher levels to
 	 * control this. For now if the tables use sme_set then so do the ptes.
 	 */
-	if (pt_feature(common, PT_FEAT_X86_64_AMD_ENCRYPT_TABLES))
+	if (pt_feature(common, PT_FEAT_X86_64_AMD_ENCRYPT_TABLES) &&
+	    !(iommu_prot & IOMMU_MMIO))
 		pte = __sme_set(pte);
 
 	attrs->descriptor_bits = pte;
-- 
2.51.1