Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked() helper

Baolu Lu
2025-08-15T05:30:15Z

On 8/12/25 06:59, Nicolin Chen wrote:
> There is a need to attach a PCI device that's under a reset to temporally
> the blocked domain (i.e. detach it from its previously attached domain),
> and then to reattach it back to its previous domain (i.e. detach it from
> the blocked domain) after reset.
>
> During the reset stage, there can be races from other attach/detachment.
> To solve this, a per-gdev reset flag will be introduced so that all the
> attach functions will bypass the driver-level attach_dev callbacks, but
> only update the group->domain pointer. The reset recovery procedure will
> attach directly to the cached pointer so things will be back to normal.
>
> On the other hand, the iommu_get_domain_for_dev() API always returns the
> group->domain pointer, and several IOMMMU drivers call this API in their
> attach_dev callback functions to get the currently attached domain for a
> device, which will be broken for the recovery case mentioned above:
> 1. core asks the driver to attach dev from blocked to group->domain
> 2. driver attaches dev from group->domain to group->domain
>
> So, iommu_get_domain_for_dev() should check the gdev flag and return the
> blocked domain if the flag is set. But the caller of this API could hold
> the group->mutex already or not, making it difficult to add the lock.
>
> Introduce a new iommu_get_domain_for_dev_locked() helper to be used by
> those drivers in a context that is already under the protection of the
> group->mutex, e.g. those attach_dev callback functions. And roll out the
> new helper to all the existing IOMMU drivers.

Given that iommu_group->mutex is transparent to the iommu driver, how
about

/*
* Called only by iommu drivers in the callback context where
* group->mutex has already been held by the core.
*/
struct iommu_domain *iommu_get_domain_for_dev_internal(struct device *dev)
{
...
lockdep_assert_held(&group->mutex);
...
}

?

>
> Add a lockdep_assert_not_held to the existing iommu_get_domain_for_dev()
> to note that it would be only used outside the group->mutex.
>
> Signed-off-by: Nicolin Chen<nicolinc@nvidia.com>

Thanks,
baolu