Re: [PATCH 3/4] iommu: Add support to change default domain of an iommu_group

[Joerg Roedel <joro@8bytes.org>]

On Tue, Aug 20, 2019 at 07:42:25PM -0700, Sai Praneeth Prakhya wrote:
> +	/*
> +	 * iommu_domain_alloc() takes "struct bus_type" as an argument which is
> +	 * a member in "struct device". Changing a group's default domain type
> +	 * deals at iommu_group level rather than device level and hence there
> +	 * is no straight forward way to get "bus_type" of an iommu_group that
> +	 * could be passed to iommu_domain_alloc(). So, instead of directly
> +	 * calling iommu_domain_alloc(), use iommu_ops from previous default
> +	 * domain.
> +	 */
> +	if (!prev_domain || !prev_domain->ops ||
> +	    !prev_domain->ops->domain_alloc || !type)
> +		return -EINVAL;

Hmm, this isn't really nice and clean, but I understand why you need it.
I will think about a better way to get iommu_ops here.

> +free_prev_domain:
> +	/*
> +	 * Free the existing default domain and replace it with the newly
> +	 * created default domain. No need to set group->domain because
> +	 * __iommu_attach_group() already does it on success.
> +	 */
> +	iommu_domain_free(prev_domain);
> +	group->default_domain = new_domain;
> +	return 0;

It isn't obvious to me from this patch, how to are the dma_ops updated
when the default domain changes between identity and dma?

> +	/* Check if any device in the group still has a driver binded to it */
> +	if (iommu_group_for_each_dev(group, NULL, is_driver_binded)) {
> +		pr_err("Active drivers exist for devices in the group\n");
> +		return -EBUSY;
> +	}

This is racy with device driver probing code. Unfortunatly there is no
clean way out of that either, locking all devices in the group and then
do the re-attach will introduce a lock-inversion with group->mutex. But
please put a comment here saying that this might race with device driver
probing.


Regards,

	Joerg
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