RE: [PATCH v2 08/10] iommufd/device: Use iommu_group_replace_domain()

From: Nicolin Chen nicolinc@nvidia.com Sent: Tuesday, February 14, 2023 7:00 PM

On Fri, Feb 10, 2023 at 02:11:23AM +0000, Tian, Kevin wrote:

...

My confusion is that we have different flows between detach/attach and replace.

today with separate detach+attach we have following flow:

    Remove device from current hwpt;
    if (last_device in hwpt) {
            Remove hwpt domain from current iopt;
            if (last_device in group)
                    detach group from hwpt domain;
    }

    if (first device in group) {
            attach group to new hwpt domain;
            if (first_device in hwpt)
                    Add hwpt domain to new iopt;
    Add device to new hwpt;

but replace flow is different on the detach part:

    if (first device in group) {
            replace group's domain from current hwpt to new hwpt;
            if (first_device in hwpt)
                    Add hwpt domain to new iopt;
    }

    Remove device from old hwpt;
    if (last_device in old hwpt)
            Remove hwpt domain from old iopt;

    Add device to new hwpt;

I'm yet to figure out whether we have sufficient lock protection to prevent other paths from using old iopt/hwpt to find the device which is already attached to a different domain.

With Jason's new series, the detach() routine is lighter now.

I wonder if it'd be safer now to do the detach() call after iommu_group_replace_domain()?

yes, looks so.