[PATCH v2 0/1] KVM: s390: pv: fix asynchronous teardown for small VMs

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On machines without the Destroy Secure Configuration Fast UVC, the
topmost level of page tables is set aside and freed asynchronously
as last step of the asynchronous teardown.

Each gmap has a host_to_guest radix tree mapping host (userspace)
addresses (with 1M granularity) to gmap segment table entries (pmds).

If a guest is smaller than 2GB, the topmost level of page tables is the
segment table (i.e. there are only 2 levels). Replacing it means that
the pointers in the host_to_guest mapping would become stale and cause
all kinds of nasty issues.

This patch fixes the issue by disallowing asynchronous teardown for
guests with only 2 levels of page tables. Userspace should (and already
does) try using the normal destroy if the asynchronous one fails.

Update s390_replace_asce so it refuses to replace segment type ASCEs.

v1->v2:
After talking with Marc, I decided to throw away most of the patch and
instead simply refuse to prepare for asynchronous teardown if the VM has a
segment type ASCE.


Claudio Imbrenda (1):
  KVM: s390: pv: fix asynchronous teardown for small VMs

 arch/s390/kvm/pv.c  | 5 +++++
 arch/s390/mm/gmap.c | 7 +++++++
 2 files changed, 12 insertions(+)

-- 
2.40.0