On Fri, 31 Jan 2020 15:51:25 -0800 Jacob Pan <[email protected]> wrote:
> Hi Alex, > Sorry I missed this part in the previous reply. Comments below. > > On Wed, 29 Jan 2020 15:19:51 -0700 > Alex Williamson <[email protected]> wrote: > > > Also, is the 12-bytes of padding in struct iommu_gpasid_bind_data > > excessive with this new versioning scheme? Per rule #2 I'm not sure > > if we're allowed to repurpose those padding bytes, > We can still use the padding bytes as long as there is a new flag bit > to indicate the validity of the new filed within the padding. > I should have made it clear in rule #2 when mentioning the flags bits. > Should define what extension constitutes. > How about this? > " > * 2. Data structures are open to extension but closed to modification. > * Extension should leverage the padding bytes first where a new > * flag bit is required to indicate the validity of each new member. > * The above rule for padding bytes also applies to adding new union > * members. > * After padding bytes are exhausted, new fields must be added at the > * end of each data structure with 64bit alignment. Flag bits can be > * added without size change but existing ones cannot be altered. > * > " > So if we add new field by doing re-purpose of padding bytes, size > lookup result will remain the same. New code would recognize the new > flag, old code stays the same. > > VFIO layer checks for UAPI compatibility and size to copy, version > sanity check and flag usage are done in the IOMMU code. > > > but if we add > > fields to the end of the structure as the scheme suggests, we're > > stuck with not being able to expand the union for new fields. > Good point, it does sound contradictory. I hope the rewritten rule #2 > address that. > Adding data after the union should be extremely rare. Do you see any > issues with the example below? > > offsetofend() can still find the right size. > e.g. > V1 > struct iommu_gpasid_bind_data { > __u32 version; > #define IOMMU_PASID_FORMAT_INTEL_VTD 1 > __u32 format; > #define IOMMU_SVA_GPASID_VAL (1 << 0) /* guest PASID valid */ > __u64 flags; > __u64 gpgd; > __u64 hpasid; > __u64 gpasid; > __u32 addr_width; > __u8 padding[12]; > /* Vendor specific data */ > union { > struct iommu_gpasid_bind_data_vtd vtd; > }; > }; > > const static int > iommu_uapi_data_size[NR_IOMMU_UAPI_TYPE][IOMMU_UAPI_VERSION] = { /* > IOMMU_UAPI_BIND_GPASID */ {offsetofend(struct iommu_gpasid_bind_data, > vtd)}, ... > }; > > V2, Add new_member at the end (forget padding for now). > struct iommu_gpasid_bind_data { > __u32 version; > #define IOMMU_PASID_FORMAT_INTEL_VTD 1 > __u32 format; > #define IOMMU_SVA_GPASID_VAL (1 << 0) /* guest PASID valid */ > #define IOMMU_NEW_MEMBER_VAL (1 << 1) /* new member added */ > __u64 flags; > __u64 gpgd; > __u64 hpasid; > __u64 gpasid; > __u32 addr_width; > __u8 padding[12]; > /* Vendor specific data */ > union { > struct iommu_gpasid_bind_data_vtd vtd; > }; > __u64 new_member; > }; > const static int > iommu_uapi_data_size[NR_IOMMU_UAPI_TYPE][IOMMU_UAPI_VERSION] = { /* > IOMMU_UAPI_BIND_GPASID */ > {offsetofend(struct iommu_gpasid_bind_data, > vtd), offsetofend(struct iommu_gpasid_bind_data,new_member)}, > > }; > > V3, Add smmu to the union,larger than vtd > > struct iommu_gpasid_bind_data { > __u32 version; > #define IOMMU_PASID_FORMAT_INTEL_VTD 1 > #define IOMMU_PASID_FORMAT_INTEL_SMMU 2 > __u32 format; > #define IOMMU_SVA_GPASID_VAL (1 << 0) /* guest PASID valid */ > #define IOMMU_NEW_MEMBER_VAL (1 << 1) /* new member added */ > #define IOMMU_SVA_SMMU_SUPP (1 << 2) /* SMMU data supported */ > __u64 flags; > __u64 gpgd; > __u64 hpasid; > __u64 gpasid; > __u32 addr_width; > __u8 padding[12]; > /* Vendor specific data */ > union { > struct iommu_gpasid_bind_data_vtd vtd; > struct iommu_gpasid_bind_data_smmu smmu; > }; > __u64 new_member; > }; > const static int > iommu_uapi_data_size[NR_IOMMU_UAPI_TYPE][IOMMU_UAPI_VERSION] = { > /* IOMMU_UAPI_BIND_GPASID */ > {offsetofend(struct iommu_gpasid_bind_data,vtd), > offsetofend(struct iommu_gpasid_bind_data, new_member), > offsetofend(struct iommu_gpasid_bind_data, new_member)}, > ... > }; > How are you not breaking rule #3, "Versions are backward compatible" with this? If the kernel is at version 3 and userspace is at version 2 then new_member exists at different offsets of the structure. The kernels iommu_uapi_data_size for V2 changed between version 2 and 3. Thanks, Alex _______________________________________________ iommu mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/iommu
