Re: [PATCH v2 10/19] iommufd/viommu: Add vdev_id helpers for IOMMU drivers

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 05, 2024 at 10:53:31AM -0700, Nicolin Chen wrote:
> On Thu, Sep 05, 2024 at 01:14:15PM -0300, Jason Gunthorpe wrote:
> > On Tue, Aug 27, 2024 at 09:59:47AM -0700, Nicolin Chen wrote:
> > > Driver can call the iommufd_viommu_find_device() to find a device pointer
> > > using its per-viommu virtual ID. The returned device must be protected by
> > > the pair of iommufd_viommu_lock/unlock_vdev_id() function.
> > > 
> > > Put these three functions into a new viommu_api file, to build it with the
> > > IOMMUFD_DRIVER config.
> > > 
> > > Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> > > ---
> > >  drivers/iommu/iommufd/Makefile     |  2 +-
> > >  drivers/iommu/iommufd/viommu_api.c | 39 ++++++++++++++++++++++++++++++
> > >  include/linux/iommufd.h            | 16 ++++++++++++
> > >  3 files changed, 56 insertions(+), 1 deletion(-)
> > >  create mode 100644 drivers/iommu/iommufd/viommu_api.c
> > 
> > I still think this is better to just share the struct content with the
> > driver, eventually we want to do this anyhow as the driver will
> > want to use container_of() techniques to reach its private data.
> 
> In my mind, exposing everything to the driver is something that
> we have to (for driver-managed structures) v.s. we want to...
> Even in that case, a driver actually only need to know the size
> of the core structure, without touching what's inside(?).
> 
> I am a bit worried that drivers would abuse the content in the
> core-level structure.. Providing a set of API would encourage
> them to keep the core structure intact, hopefully..

This is always a tension in the kernel. If the core apis can be nice
and tidy then it is a reasonable direction

But here I think we've cross some threshold where the APIs are
complex, want to be inlined and really we just want to expose data not
APIs to drivers.

> > No need for this lock, xa_load is rcu safe against concurrent writer
> 
> I see iommufd's device.c and main.c grab xa_lock before xa_load?

That is not to protect the xa_load, it is to protect the lifetime of
pointer it returns

Jason