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

[Alex Williamson <alex.williamson@redhat.com>]

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