Re: [PATCH v2 8/8] s390/vfio-ap: Fix hot-unplug skipped when last AP adapter or domain removed

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 8/10/26 3:22 PM, Anthony Krowiak wrote:
> The vfio_ap_mdev_hot_unplug_cfg() function uses the return value of
> bitmap_andnot() to determine whether the guest APCB needs to be updated.
> However, bitmap_andnot() returns false when the resulting destination
> bitmap is empty. This means that if the only adapter, domain or control
> domain assigned to an mdev is removed from the host's AP confikguration,

s/confikguration/configuration/

> the bit is correctly cleared from the shadow APCB, but bitmap_andnot()
> returns false because the result is an empty bitmap. Consequently,
> do_hotplug remains 0 and vfio_ap_mdev_update_guest_apcb() is never called,
> leaving the KVM guest with stale hardware access to the unplugged AP
> devices.
> 
> Fix this by replacing the bitmap_andnot() return value check with
> bitmap_intersects() to determine whether the shadow APCB actually
> overlaps with the removal mask. If there is an intersection, call
> bitmap_andnot() solely for its side effect of clearing the bits, then
> unconditionally set do_hotplug to trigger the guest APCB update.
> 
> Fixes: eeb386aeb5b7c ("s390/vfio-ap: handle config changed and scan complete notification")
> Cc: stable@vger.kernel.org
> Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>

Also please look at the one other pre-existing finding against this
patch, another potential follow-on.