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

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Message-ID: <20240408140548.GO5383@nvidia.com>
Date: Mon, 8 Apr 2024 11:05:48 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Jean-Philippe Brucker <jean-philippe@...aro.org>,
	Nicolin Chen <nicolinc@...dia.com>, Yi Liu <yi.l.liu@...el.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Joel Granados <j.granados@...sung.com>, iommu@...ts.linux.dev,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/9] iommu: Introduce domain attachment handle

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@...dia.com>
> > > Signed-off-by: Jason Gunthorpe<jgg@...dia.com>
> > > Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
> > > ---
> > >   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?

> +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.

> +       struct iopf_group *group;
> +       unsigned long index;
> +
> +       mutex_lock(&fault->mutex);
> +       xa_for_each(&idev->faults, index, group) {
> +               xa_erase(&idev->faults, index);
> +               iopf_group_response(group, IOMMU_PAGE_RESP_INVALID);
> +       }
> +       mutex_unlock(&fault->mutex);

This makes sense, yes..

>  void iommufd_fault_domain_detach_dev(struct iommufd_hw_pagetable *hwpt,
>                                      struct iommufd_device *idev)
>  {
> +       struct iommufd_fault *fault = hwpt->fault;
> +       struct iommu_attach_handle *handle;
> +
>         if (WARN_ON(!hwpt->fault_capable))
>                 return;
> 
> +       handle = iommu_attach_handle_get(idev->igroup->group,
> IOMMU_NO_PASID);
>         iommu_detach_group(hwpt->domain, idev->igroup->group);
>         iommufd_fault_iopf_disable(idev);

But is this right? Couldn't there be PASID's doing PRI?

Jason