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

[Baolu Lu <baolu.lu@linux.intel.com>]

On 12/19/24 13:06, Nicolin Chen wrote:
> 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 for the explanation. Maybe I am a bit over-considering here.

Initially, my understanding is to report a virtual device ID when the
object originates from a vIOMMU, and an iommufd device ID otherwise.

However, considering page fault scenarios, which are self-contained but
linked to a hardware page table (hwpt), introduces ambiguity. Hwpt can
be created with or without a vIOMMU. This raises the question: should
the page fault message always report the iommufd device ID, or should
the reporting depend on whether the hwpt was created from a vIOMMU?

Thanks,
baolu