Re: [PATCH 2/2] iommu/vt-d: Use device rbtree in iopf reporting path

[Jason Gunthorpe]

On Sun, Feb 18, 2024 at 03:02:00PM +0800, Baolu Lu wrote:
> A device hot removing goes through at least the following steps:
> 
> - Disable PRI.
> - Drain all outstanding I/O page faults.
> - Stop DMA.
> - Unload the device driver.
> - Call iommu_release_device() upon the BUS_NOTIFY_REMOVED_DEVICE event.
> 
> This sequence ensures that a device cannot generate an I/O page fault
> after PRI has been disabled. So in reality it's impossible for a device
> to generate an I/O page fault before disabling PRI and then go through
> the long journey to reach iommu_release_device() before
> iopf_get_dev_fault_param() is called in page fault interrupt handling
> thread.

Why is this impossible? Seems like a classic race..

Flush the HW page fault queue as part of the above to ensure there is
no concurrent iopf_get_dev_fault_param() on the now PRI disabled BDF.

> Considering this behavior, adding a comment to the code explaining the
> sequence and removing put_device() may be a simpler solution?

A comment is definitely needed

Jason