Re: [PATCH 3/3] iommu/uapi: Add helper function for size lookup

Alex Williamson
2020-02-03T18:27:22Z

On Fri, 31 Jan 2020 15:51:25 -0800
Jacob Pan <jacob.jun.pan@linux.intel.com> 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 <alex.williamson@redhat.com> 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