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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Jason Gunthorpe <jgg@ziepe.ca>,
        Anthony Krowiak <akrowiak@linux.ibm.com>
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        "Liu, Yi L" <yi.l.liu@intel.com>
Subject: RE: [PATCH 01/15] vfio: Add helpers for unifying vfio_device life
 cycle
Thread-Topic: [PATCH 01/15] vfio: Add helpers for unifying vfio_device life
 cycle
Thread-Index: AQHYufpgD2KvzjTjxECQ8qBhswxZGK3HeMwAgAAYbgCAAPkekA==
Date:   Wed, 31 Aug 2022 06:03:54 +0000
Message-ID: <BN9PR11MB527696C9C458E1DE6E60B3A98C789@BN9PR11MB5276.namprd11.prod.outlook.com>
References: <20220827171037.30297-1-kevin.tian@intel.com>
 <20220827171037.30297-2-kevin.tian@intel.com>
 <907c54c6-7f5b-77f3-c284-45604c60c12e@linux.ibm.com>
 <Yw4oUL33TbJK6inc@ziepe.ca>
In-Reply-To: <Yw4oUL33TbJK6inc@ziepe.ca>
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> From: Jason Gunthorpe <jgg@ziepe.ca>
> Sent: Tuesday, August 30, 2022 11:10 PM
>=20
> On Tue, Aug 30, 2022 at 09:42:42AM -0400, Anthony Krowiak wrote:
>=20
> > > +/*
> > > + * Alloc and initialize vfio_device so it can be registered to vfio
> > > + * core.
> > > + *
> > > + * Drivers should use the wrapper vfio_alloc_device() for allocation=
.
> > > + * @size is the size of the structure to be allocated, including any
> > > + * private data used by the driver.
> >
> >
> > It seems the purpose of the wrapper is to ensure that the object being
> > allocated has as its first field a struct vfio_device object and to ret=
urn
> > its container. Why not just make that a requirement for this function -
> > which I would rename vfio_alloc_device - and document it in the prologu=
e?
> > The caller can then cast the return pointer or use container_of.
>=20
> There are three fairly common patterns for this kind of thing
>=20
> 1) The caller open codes everything:
>=20
>    driver_struct =3D kzalloc()
>    core_init(&driver_struct->core)
>=20
> 2) Some 'get priv' / 'get data' is used instead of container_of():
>=20
>    core_struct =3D core_alloc(sizeof(*driver_struct))
>    driver_struct =3D core_get_priv(core_struct)
>=20
> 3) The allocations and initialization are consolidated in the core,
>    but we continue to use container_of()
>=20
>    driver_struct =3D core_alloc(typeof(*driver_struct))
>=20
> #1 has a general drawback that people routinely mess up the lifecycle
> model and get really confused about when to do kfree() vs put(),
> creating bugs.
>=20
> #2 has a general drawback of not using container_of() at all, and being
> a bit confusing in some cases
>=20
> #3 has the general drawback of being a bit magical, but solves 1 and
> 2's problems.
>=20
> I would not fix the struct layout without the BUILD_BUG_ON because
> someone will accidently change the order and that becomes a subtle
> runtime error - so at a minimum the wrapper macro has to exist to
> check that.

Agree. And gvt happened to hit this BUILD_BUG_ON when this series
was being worked on.

>=20
> If you want to allow a dynamic struct layout and avoid the pitfall of
> exposing the user to kalloc/kfree, then you still need the macro, and
> it does some more complicated offset stuff.
>=20
> Having the wrapper macro be entirely type safe is appealing and
> reduces code in the drivers, IMHO. Tell it what type you are initing
> and get back init'd memory for that type that you always, always free
> with a put operation.
>=20
> Jason