The SMMUv3.1-XNX feature is mandatory for an SMMUv3.1 if S2P is supported, so we should theoretically have implemented it as part of the recent S2P work. Fortunately, for us the implementation is a no-op.
This feature is about interpretation of the stage 2 page table descriptor XN bits, which control execute permissions. For QEMU, the permission bits passed to an IOMMU (via MemTxAttrs and IOMMUAccessFlags) only indicate read and write; we do not distinguish data reads from instruction reads outside the CPU proper. In the SMMU architecture's terms, our interconnect between the client device and the SMMU doesn't have the ability to convey the INST attribute, and we therefore use the default value of "data" for this attribute. We also do not support the bits in the Stream Table Entry that can override the on-the-bus transaction attribute permissions (we do not set SMMU_IDR1.ATTR_PERMS_OVR=1). These two things together mean that for our implementation, it never has to deal with transactions with the INST attribute, and so it can correctly ignore the XN bits entirely. So we already implement FEAT_XNX's "XN field is now 2 bits, not 1" behaviour to the extent that we need to. Patches 1 and 2 in this series do a little bit of tidy up on the ID register bit code. Patch 3 is the one-liner to advertise SMMUv3.1-XNX in the ID register. thanks -- PMM Peter Maydell (3): hw/arm/smmuv3: Update ID register bit field definitions hw/arm/smmuv3: Sort ID register setting into field order hw/arm/smmuv3: Advertise SMMUv3.1-XNX feature hw/arm/smmuv3-internal.h | 38 ++++++++++++++++++++++++++++++++++++++ hw/arm/smmuv3.c | 5 +++-- 2 files changed, 41 insertions(+), 2 deletions(-) -- 2.34.1
