RE: [PATCH v6 07/10] iommufd: Fault-capable hwpt attach/detach/replace

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Monday, May 27, 2024 12:05 PM
> 
> Add iopf-capable hw page table attach/detach/replace helpers. The pointer
> to iommufd_device is stored in the domain attachment handle, so that it
> can be echo'ed back in the iopf_group.

this message needs an update. now the device pointer is not in the
attach handle.

and there worths a explanation about VF in the commit msg.

> @@ -376,7 +377,10 @@ int iommufd_hw_pagetable_attach(struct
> iommufd_hw_pagetable *hwpt,
>  	 * attachment.
>  	 */
>  	if (list_empty(&idev->igroup->device_list)) {
> -		rc = iommu_attach_group(hwpt->domain, idev->igroup->group);
> +		if (hwpt->fault)
> +			rc = iommufd_fault_domain_attach_dev(hwpt, idev);
> +		else
> +			rc = iommu_attach_group(hwpt->domain, idev-
> >igroup->group);

Does it read better to have a iommufd_attach_device() wrapper with
above branches handled internally?

> 
> +static int iommufd_fault_iopf_enable(struct iommufd_device *idev)
> +{
> +	struct device *dev = idev->dev;
> +	int ret;
> +
> +	/*
> +	 * Once we turn on PCI/PRI support for VF, the response failure code
> +	 * could not be forwarded to the hardware due to PRI being a shared

you could but just doing so is incorrect. 😊

s/could/should/

> +	 * resource between PF and VFs. There is no coordination for this
> +	 * shared capability. This waits for a vPRI reset to recover.
> +	 */

this may go a bit further to talk about supporting it requires an emulation
in iommufd (i.e. pause any further fault delivery until vPRI reset). It is a
future work so disable it for VF at this point.

> +void iommufd_fault_domain_detach_dev(struct iommufd_hw_pagetable
> *hwpt,
> +				     struct iommufd_device *idev)
> +{
> +	struct iommufd_attach_handle *handle;
> +
> +	handle = iommufd_device_get_attach_handle(idev);
> +	iommu_detach_group_handle(hwpt->domain, idev->igroup->group);
> +	iommufd_auto_response_faults(hwpt, handle);
> +	iommufd_fault_iopf_disable(idev);
> +	kfree(handle);

this assumes that the detach callback of the iommu driver needs to drain
in-fly page requests so no further reference to handle or queue new req
to the deliver queue when it returns, otherwise there is a use-after-free
race or stale requests in the fault queue which auto response doesn't
cleanly handle.

iirc previous discussion reveals that only intel-iommu driver guarantees
that behavior. In any case this should be documented to avoid this being
used in a non-conforming iommu driver.

If I misunderstood, some comment why no race in this window is also
appreciated. 😊

> +}
> +
> +static int __fault_domain_replace_dev(struct iommufd_device *idev,
> +				      struct iommufd_hw_pagetable *hwpt,
> +				      struct iommufd_hw_pagetable *old)
> +{
> +	struct iommufd_attach_handle *handle, *curr = NULL;
> +	int ret;
> +
> +	if (old->fault)
> +		curr = iommufd_device_get_attach_handle(idev);
> +
> +	if (hwpt->fault) {
> +		handle = kzalloc(sizeof(*handle), GFP_KERNEL);
> +		if (!handle)
> +			return -ENOMEM;
> +
> +		handle->handle.domain = hwpt->domain;
> +		handle->idev = idev;
> +		ret = iommu_replace_group_handle(idev->igroup->group,
> +						 hwpt->domain, &handle-
> >handle);
> +	} else {
> +		ret = iommu_replace_group_handle(idev->igroup->group,
> +						 hwpt->domain, NULL);
> +	}
> +
> +	if (!ret && curr) {
> +		iommufd_auto_response_faults(old, curr);
> +		kfree(curr);
> +	}

In last version you said auto response is required only when switching
from fault-capable old to fault-incapable new. But above code doesn't
reflect that description?