Re: [PATCH v9 3/7] iommu/uapi: Introduce enum type for PASID data format

Jacob Pan
2020-09-22T20:22:10Z

Hi Joerg,

I sent out v10 with Randy's comments addressed but I didn't change this
patch. Does my explanation below make sense? I am hoping to make it in
v5.10 since many other pieces depend on it, your guidance is much
appreciated.

Jacob

On Fri, 18 Sep 2020 10:11:08 -0700, Jacob Pan
<jacob.jun.pan@linux.intel.com> wrote:

> Hi Joerg,
>
> On Fri, 18 Sep 2020 11:44:50 +0200, Joerg Roedel <joro@8bytes.org> wrote:
>
> > On Fri, Sep 11, 2020 at 02:57:52PM -0700, Jacob Pan wrote:
> > > There can be multiple vendor-specific PASID data formats used in UAPI
> > > structures. This patch adds enum type with a last entry which makes
> > > range checking much easier.
> >
> > But it also makes it much easier to screw up the numbers (which are ABI)
> > by inserting a new value into the middle. I prefer defines here, or
> > alternativly BUILD_BUG_ON() checks for the numbers.
> >
> I am not following, the purpose of IOMMU_PASID_FORMAT_LAST *is* for
> preparing the future insertion of new value into the middle.
> The checking against IOMMU_PASID_FORMAT_LAST is to protect ABI
> compatibility by making sure that out of range format are rejected in all
> versions of the ABI.
> For example, in v5.10, ABI has IOMMU_PASID_FORMAT_LAST = 2, then user data
> with format = 2 will be rejected. So this user app will not work or
> released.
>
> Now say in v5.11, we add one more format in the middle and set
> IOMMU_PASID_FORMAT_LAST = 3. Then user data with the new format = 2 can
> be supported.
>
> Without the checking for IOMMU_PASID_FORMAT_LAST, at v5.10 time the user
> binary may succeed and become legacy binary that we cannot break in v5.11.
> This renders format = 2 unusable for v5.11.
>
> I thought enum makes it less susceptible to programming errors than
> defines by making sure the ascending order. I might have missed your
> point, could you elaborate?
>
> > Regards,
> >
> > Joerg
> >
> > >
> > > Suggested-by: Alex Williamson <alex.williamson@redhat.com>
> > > Reviewed-by: Eric Auger <eric.auger@redhat.com>
> > > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > ---
> > > include/uapi/linux/iommu.h | 8 ++++++--
> > > 1 file changed, 6 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/uapi/linux/iommu.h b/include/uapi/linux/iommu.h
> > > index b42acc8fe007..7cc6ee6c41f7 100644
> > > --- a/include/uapi/linux/iommu.h
> > > +++ b/include/uapi/linux/iommu.h
> > > @@ -298,11 +298,16 @@ struct iommu_gpasid_bind_data_vtd {
> > > IOMMU_SVA_VTD_GPASID_PCD |
> > > \ IOMMU_SVA_VTD_GPASID_PWT)
> > >
> > > +enum iommu_pasid_data_format {
> > > + IOMMU_PASID_FORMAT_INTEL_VTD = 1,
> > > + IOMMU_PASID_FORMAT_LAST,
> > > +};
> > > +
> > > /**
> > > * struct iommu_gpasid_bind_data - Information about device and guest
> > > PASID binding
> > > * @argsz: User filled size of this data
> > > * @version: Version of this data structure
> > > - * @format: PASID table entry format
> > > + * @format: PASID table entry format of enum
> > > iommu_pasid_data_format type
> > > * @flags: Additional information on guest bind request
> > > * @gpgd: Guest page directory base of the guest mm to bind
> > > * @hpasid: Process address space ID used for the guest mm in
> > > host IOMMU @@ -321,7 +326,6 @@ struct iommu_gpasid_bind_data {
> > > __u32 argsz;
> > > #define IOMMU_GPASID_BIND_VERSION_1 1
> > > __u32 version;
> > > -#define IOMMU_PASID_FORMAT_INTEL_VTD 1
> > > __u32 format;
> > > __u32 addr_width;
> > > #define IOMMU_SVA_GPASID_VAL (1 << 0) /* guest PASID valid */
> > > --
> > > 2.7.4
>
>
> Thanks,
>
> Jacob

Thanks,

Jacob