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

[Nicolin Chen]

On Wed, Sep 11, 2024 at 08:11:03PM -0300, Jason Gunthorpe wrote:
> 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@xxxxxxxxxx>
> > > > ---
> > > >  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.

OK. I'll think of a rework. And might need another justification
for a DEFAULT type of vIOMMU object to fit in.

> > > 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

I see. I'd drop it.

Thanks
Nicolin