Re: [PATCH v4 1/9] iommu: Introduce domain attachment handle

[Baolu Lu]

On 4/8/24 10:05 PM, Jason Gunthorpe wrote:
> On Sat, Apr 06, 2024 at 12:34:14PM +0800, Baolu Lu wrote:
> > On 4/3/24 7:58 PM, Jason Gunthorpe wrote:
> > > On Wed, Apr 03, 2024 at 09:15:11AM +0800, Lu Baolu wrote:
> > > > Currently, when attaching a domain to a device or its PASID, domain is
> > > > stored within the iommu group. It could be retrieved for use during the
> > > > window between attachment and detachment.
> > > >
> > > > With new features introduced, there's a need to store more information
> > > > than just a domain pointer. This information essentially represents the
> > > > association between a domain and a device. For example, the SVA code
> > > > already has a custom struct iommu_sva which represents a bond between
> > > > sva domain and a PASID of a device. Looking forward, the IOMMUFD needs
> > > > a place to store the iommufd_device pointer in the core, so that the
> > > > device object ID could be quickly retrieved in the critical fault handling
> > > > path.
> > > >
> > > > Introduce domain attachment handle that explicitly represents the
> > > > attachment relationship between a domain and a device or its PASID.
> > > > A caller-specific data field can be used by the caller to store additional
> > > > information beyond a domain pointer, depending on its specific use case.
> > > >
> > > > Co-developed-by: Jason Gunthorpe<jgg@xxxxxxxxxx>
> > > > Signed-off-by: Jason Gunthorpe<jgg@xxxxxxxxxx>
> > > > Signed-off-by: Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
> > > > ---
> > > >    drivers/iommu/iommu-priv.h |   9 +++
> > > >    drivers/iommu/iommu.c      | 158 +++++++++++++++++++++++++++++++++----
> > > >    2 files changed, 153 insertions(+), 14 deletions(-)
> > > >
> > > > diff --git a/drivers/iommu/iommu-priv.h b/drivers/iommu/iommu-priv.h
> > > > index 5f731d994803..08c0667cef54 100644
> > > > --- a/drivers/iommu/iommu-priv.h
> > > > +++ b/drivers/iommu/iommu-priv.h
> > > > @@ -28,4 +28,13 @@ void iommu_device_unregister_bus(struct iommu_device *iommu,
> > > >    				 const struct bus_type *bus,
> > > >    				 struct notifier_block *nb);
> > > > +struct iommu_attach_handle {
> > > > +	struct iommu_domain		*domain;
> > > > +	refcount_t			users;
> > > I don't understand how the refcounting can be generally useful. There
> > > is no way to free this:
> > >
> > > > +	void				*priv;
> > > When the refcount goes to zero.
> > This field is set by the caller, so the caller ensures that the pointer
> > can only be freed after iommu domain detachment. For iopf, the caller
> > should automatically respond to all outstanding iopf's in the domain
> > detach path.
> >
> > In the sva case, which uses the workqueue to handle iopf,
> > flush_workqueue() is called in the domain detach path to ensure that all
> > outstanding iopf's are completed before detach completion.
> Which is back to what is the point of the refcount at all?

Yeah, refcount is not generally useful. It's context-specific, so it
needs to move out of the core.

The SVA code needs refcount because it allows multiple attachments
between a SVA domain and a PASID. This is not a common case.

> > +static void iommufd_auto_response_handle(struct iommufd_fault *fault,
> > +                                        struct iommu_attach_handle *handle)
> > +{
> > +       struct iommufd_device *idev = handle->priv;
> The caller already has the iommufd_device, don't need the handler.

Yes.

Best regards,
baolu