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

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

On Mon, 3 Feb 2020 11:27:08 -0700
Alex Williamson <alex.williamson@redhat.com> wrote:

> 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,
> 
You are right. if we want to add new member to the end of the structure
as well as expanding union, I think we have to fix the size of the
union. Would this work? (just an example for one struct)


@@ -344,6 +348,11 @@ struct iommu_gpasid_bind_data_vtd {
  * @gpasid:    Process address space ID used for the guest mm in guest
IOMMU
  * @addr_width:        Guest virtual address width
  * @padding:   Reserved for future use (should be zero)
+ * @dummy      Reserve space for vendor specific data in the union. New
+ *             members added to the union cannot exceed the size of
dummy.
+ *             The fixed size union is needed to allow further
expansion
+ *             after the end of the union while still maintain backward
+ *             compatibility.
  * @vtd:       Intel VT-d specific data
  *
  * Guest to host PASID mapping can be an identity or non-identity,
where guest @@ -365,6 +374,7 @@ struct iommu_gpasid_bind_data {
        __u8  padding[12];
        /* Vendor specific data */
        union {
+               __u8 dummy[128];
                struct iommu_gpasid_bind_data_vtd vtd;
        };
 };

> Alex
> 

[Jacob Pan]
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