Re: [PATCH v3 07/14] iommufd/viommu: Add iommufd_viommu_get_vdev_id helper

[Nicolin Chen <nicolinc@nvidia.com>]

On Thu, Dec 19, 2024 at 10:05:53AM +0800, Baolu Lu wrote:
> On 12/18/24 13:00, Nicolin Chen wrote:
> > This is a reverse search v.s. iommufd_viommu_find_dev, as drivers may want
> > to convert a struct device pointer (physical) to its virtual device ID for
> > an event injection to the user space VM.
> > 
> > Again, this avoids exposing more core structures to the drivers, than the
> > iommufd_viommu alone.
> > 
> > Signed-off-by: Nicolin Chen<nicolinc@nvidia.com>
> > ---
> >   include/linux/iommufd.h        |  8 ++++++++
> >   drivers/iommu/iommufd/driver.c | 20 ++++++++++++++++++++
> >   2 files changed, 28 insertions(+)
> > 
> > diff --git a/include/linux/iommufd.h b/include/linux/iommufd.h
> > index b082676c9e43..ac1f1897d290 100644
> > --- a/include/linux/iommufd.h
> > +++ b/include/linux/iommufd.h
> > @@ -190,6 +190,8 @@ struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
> >   					     enum iommufd_object_type type);
> >   struct device *iommufd_viommu_find_dev(struct iommufd_viommu *viommu,
> >   				       unsigned long vdev_id);
> > +unsigned long iommufd_viommu_get_vdev_id(struct iommufd_viommu *viommu,
> > +					 struct device *dev);
> 
> Hi Nicolin,
> 
> This series overall looks good to me. But I have a question that might
> be irrelevant to this series itself.
> 
> The iommufd provides both IOMMUFD_OBJ_DEVICE and IOMMUFD_OBJ_VDEVICE
> objects. What is the essential difference between these two from
> userspace's perspective?

A quick answer is an IOMMUFD_OBJ_DEVICE being a host physical
device and an IOMMUFD_OBJ_VDEVICE being an IOMMUFD_OBJ_DEVICE
related to IOMMUFD_OBJ_VIOMMU. Two of them can be seen in two
different layers. May refer to this graph:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/userspace-api/iommufd.rst?h=v6.13-rc3#n150

> And, which object ID should the IOMMU device
> driver provide when reporting other events in the future?
>
> Currently, the IOMMUFD uAPI reports IOMMUFD_OBJ_DEVICE in the page
> fault message, and IOMMUFD_OBJ_VDEVICE (if I understand it correctly) in
> the vIRQ message. It will be more future-proof if this could be defined
> clearly.

A vIRQ is actually reported per-vIOMMU in this design. Although
in the this series the SMMU driver seems to report a per-device
vIRQ, it internally converts the vDEVICE to a virtual device ID
and packs the virtual device ID into a per-vIOMMU event:

+/**
+ * struct iommu_virq_arm_smmuv3 - ARM SMMUv3 Virtual IRQ
+ *                                (IOMMU_VIRQ_TYPE_ARM_SMMUV3)
+ * @evt: 256-bit ARM SMMUv3 Event record, little-endian.
+ *       (Refer to "7.3 Event records" in SMMUv3 HW Spec)
+ *
+ * StreamID field reports a virtual device ID. To receive a virtual IRQ for a
+ * device, a vDEVICE must be allocated via IOMMU_VDEVICE_ALLOC.
+ */
+struct iommu_virq_arm_smmuv3 {
+	__aligned_le64 evt[4];
 };

Thanks
Nicolin