Re: [RFC PATCH 18/18] ioasid: Add /dev/ioasid for userspace

Jacob Pan
2021-03-11T22:53:21Z

Hi Jason,

Thanks for the review.

On Wed, 10 Mar 2021 15:23:01 -0400, Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Sat, Feb 27, 2021 at 02:01:26PM -0800, Jacob Pan wrote:
>
> > +/* -------- IOCTLs for IOASID file descriptor (/dev/ioasid) -------- */
> > +
> > +/**
> > + * IOASID_GET_API_VERSION - _IO(IOASID_TYPE, IOASID_BASE + 0)
> > + *
> > + * Report the version of the IOASID API. This allows us to bump the
> > entire
> > + * API version should we later need to add or change features in
> > incompatible
> > + * ways.
> > + * Return: IOASID_API_VERSION
> > + * Availability: Always
> > + */
> > +#define IOASID_GET_API_VERSION _IO(IOASID_TYPE,
> > IOASID_BASE + 0)
>
> I think this is generally a bad idea, if you change the API later then
> also change the ioctl numbers and everything should work out
>
> eg use the 4th argument to IOC to specify something about the ABI
>
Let me try to understand the idea, do you mean something like this?
#define IOASID_GET_INFO _IOC(_IOC_NONE, IOASID_TYPE, IOASID_BASE + 1,
sizeof(struct ioasid_info))

If we later change the size of struct ioasid_info, IOASID_GET_INFO would be
a different ioctl number. Then we will break the existing user space that
uses the old number. So I am guessing you meant we need to have a different
name also. i.e.

#define IOASID_GET_INFO_V2 _IOC(_IOC_NONE, IOASID_TYPE, IOASID_BASE + 1,
sizeof(struct ioasid_info_v2))

We can get rid of the API version, just have individual IOCTL version.
Is that right?

> Jason

Thanks,

Jacob